I'm not a professional historian. I'm a 20-year member of a living history group (the Society for Creative Anachronism) and in my time I've made some study of medieval foods.

First thing to understand is that many very common modern foods are of New World origin and were not available to medieval Europeans. Almost everything in the nightshade family - potatoes, tomatoes, bell peppers, and hot peppers, as well as corn (maize) are all from the New World (as, incidentally, is tobacco). There is evidence of these food items entering the diets of elite Europeans towards the third quarter of the sixteenth century.

Because of a lack of modern refrigeration, foods were either preserved (often in salt) or eaten much more seasonally than today. There is some evidence to suggest that fresh fruits and vegetables were thought to be unhealthy. Prior to the Reformation and the Anglican split, everyone in western Christendom would have "fasted" on Fridays, meaning fish instead of meat products, and also no milk products. A variety of animals we would not consider to be fish (dolphins, beavers, and even certain species of ducks) were classed as fish and therefore could be eaten on Fridays. A much wider variety of animals were eaten for their meat, especially by elites, for example peacock, swan, etc.

Sweet/baking spices (ginger, cinnamon, cardamom, cloves, nutmeg, allspice, black pepper, saffron) were prestige or luxury goods so would be worked into dishes where possible. (They were not, as is commonly suggested, used to disguise the taste of rotting meat.). This tends to make medieval cuisine reminiscent of North African cuisine for the modern palate.

For a recent modern edition of a late 16th century cookbook aimed at feeding the elites, check out the Opera of Bartolomeo Scappi. I don't know the exact title off the top of my head but if you search for it on Amazon using this title, you should find it ("opera" merely means "work" in Latin and Italian)

If you're interested in medieval food trends, two books that I found to be very informative were Paul Freedman's Out of the East: Spices and the Medieval Imagination and Acquired Taste: the French Origins of Modern Cooking by T. Sarah Peterson. The short answer to your question is that medieval food was elaborately prepared and heavily spiced, with special attention given to color and presentation. Dr. Freedman notes that "medieval people had no interest in letting the basic ingredients speak for themselves." He also compares medieval food to modern middle eastern and Indian food, at least in regards to the spices used. Remember that food was wealth at this time, so culinary fashions demanded extravagance and abundance as a way to show off the wealth and power of the person hosting a feast (that's the main reason they were so heavy-handed with the spices).

As for your question regarding Charlemagne, his biographer Einhard said that he liked roasted meat and game more than anything, and he even had professional hunters who caught deer for his daily meals (chapters 22 & 24 in Life of Charlemagne). He might have liked fried chicken or cheeseburgers, had he tried them, but bacon was generally seen as a peasant dish, not nearly good enough for the Holy Roman Emperor.

Here are some past answers & discussions along similar lines:

Feel free to ask followup questions by commenting with /u/ appended to a flaired specialist's username (i.e. /u/OldWorldGlory).

Key:

c = comments
dy = days old
wk = weeks old
mo = months old
yr = years old

• [21 c | 2 yr] Royalty, variety & taste

• [30 c | 26 dy] Noble feasts, England

• [16 c | 2 yr] Noble feasts, diets & reasoning

Ian Mortimer's "The Time Traveler's Guide to Medieval England" is a good source on the subject pertaining to England, at least.

To add to the other recommendations above, I'd also suggest Susan Pinkard's A Revolution in Taste: The Rise of French Cuisine, 1650-1800, in which (amongst others) the path from early modern sweet-savoury combinations to a greater split between them in modern French cuisine is charted, for instance.