Vinegar was common and easy to make and has been around since around 5000BC, in the 14th century there was even a guild of vinegar makers and recipes have been recorded. While it would most likely have been more of the malt vinegar, made from stale beer, or apple cider vinegar, made from hard cider, style of vinegars and not white vinegar as that would have required distilling to make. Vinegar was around and in use as a food preservative. Fermentation to pickle food is also a common preservation practice and though the exact origins of the process are unknown, archaeologists believe ancient Mesopotamians pickled food as far back as 2400 B.C. So a nice tangy lacto fermented pickles certainly existed The French Medieval household book Le Ménagier de Paris (the Goodman of Paris) had recipes for pickling walnuts and various vegetables and fruits.

While sugar would not have been common and available only in the later parts of the era, it was only really available to the very rich, honey most certainly was available and was a common sweetener used in the place of sugar jam recipes from Le Menagier de Paris includes Jam recipes using honey instead of sugar.. Medieval people loved their desserts and in fact they were the first to coin the phrase so sweet tasting foods sweetened with honey would not be a surprise to them, though for the poorer folk more of a treat for a special occasion than for daily consumption.

Meat was usually salted for storage through the winter so salty flavors would not be new to people of that time either. While the peasants daily pottage sounds plain, being a dish of basically boiled grains or legumes and vegetables (usually onions and leeks as they were considered food good only for poor people), you have to remember that even the poor could grow herbs or forage for them and these would be added to the pottage, so so could include things like peppery watercress, parsley, sage, rosemary or thyme. Horseradish is easy to grow (stopping it from taking over is the hard part) or forage, stores well and grated to serve with rare meat dishes. If you've ever eaten raw horseradish, the spicy sinus clearing heat is not for the faint of heart. Radishes have a delightful tangy pepperiness and are easy to grow and garlic was as popular at the time as it is now as a flavoring agent. Fennel was also commonly grown provided a nice bright liquorice note to foods, then as it does now.

Most people had access to dairy products, though rarely in the form of milk as the production of the dairy would be turned into cheeses and butter to help preserve it and while most of it would most likely be sold, some would be kept for home consumption. Aged Cheddar cheese was first recorded as having been made in the 12th century and if you've ever had a bite of a nice aged piece of cheddar you know what a salivary gland achingly strong flavor that can have. They don't call it Ol' Bitey or Ol' Croc when branding it now a days for nothing.

In addition to what u/wwaxwork already wrote, I would like to point out that there is also one important thing that seems to be linked to a bias resulting, in my opinion, from the fact that popular culture in the Anglosphere is dominated by the portrayal of 'Middle Ages' specifically as the milieu typical of England. It should be noted however, that Europe is much larger than this, and the southern countries had ready access to e.g., citrus fruits, especially citrons (Citrus medica), cultivated there in Italia, Hispania and Greece in 2nd century BCE, and around 12th century also known in the north, usually as 'Italian fruit' or 'Italian apple'. In the time of Crusades, Europeans also brought the acid lime (Citrus aurantifolia) and lemon (Citrus limon). It is interesting that onions, a staple of Egyptian and Middle Eastern cuisine since prehistory, has been introduced to Central and Northern Europe only around the same time.

And speaking of fruits, they would most definitely fill the niche of intense, sweet and sour taste with rich flavour, especially when dried, or, in some cases, smoked. Sun-drying was a simplest form of preservation of these easily spoiling comestibles, and ripe plums or apples, having lost water and thus increasing its sugar content were certainly aromatic and sweet. Add hint of smokiness brought by the wood smoke and you can easily create a 'flavour blast' that could bring modern artificial food to shame. If it was sour one coveted, they they would need to look no further than quince, deeply sweet and astringent blackcurrants, or fresh, sour redcurrants and gooseberries. Of course, any sort of berries, with blackberries and raspberries (also lingonberries and cranberries, quite prevalent in Central and Northern parts of the continent) were quite common in season. These fruits and berries, not really suitable for drying, were often preserved in honey, combining deep and rich sweetness of honey with acidity of the fruits.

And let's not forget about drinks. Although not really popular in the north, Greek custom of adding resins and honey to wine (sometimes because of tradition, sometimes to mask the shortcomings of less expertly made wine) resulted in a distinct combination modern retsina is a descendant of. We may also take into account deeply sweet wines made of late harvest grapes, sometimes additionally frozen to reduce the water content, although these wines were quite rare and this not necessarily qualify as a part of a regular commoners' diet (but they could have been brought for special occasions in regions where vineyards were popular). In the northern areas, primarily Scandinavia and Slavic territories, good quality mead could have also proven flavourful, mixing harsh sweetness of original material with acidity and alcoholic tinge (poorly made mead is also, well, interesting in this department, if not necessarily desirable).

Of course, we can't forget about an important condiment that was quite popular since late Antiquity, first in the south, but later also in other parts of Europe. And I mean mustard. Of course, the mustard plant, chiefly the seeds, have been used in Eastern Asia as early as 40th century BCE, and it has definitely been known in 15th century BCE in Egypt, but it was used chiefly as medicine or a ground spice, not unlike pepper is used today. Possibly the first recipe for a cold mustard-based sauce comes from De re rustica written by Rutilius Taurus Aemilianus Palladius in late 4th century CE, where it is presented as a condiment made from ground mustard seeds, vinegar, honey and olive oil, what makes it pretty close to modern mustard. Between 10th and 12th century it found its way to other countries, and in early 14th century it was certainly popular and prepared in various creative ways, as evidenced by pope Clement VI who expressed his taste for 'violet mustard from Brive', made with grape peels. Lists of ingredients procured for large feasts in 12th-13th century often mention mustard being procured by the barrels, so it wasn't unpopular, and the ingredients or production process was also well within the reach of a common man.