"Boring" is very personal; "bland" less so. "Bland" can be talked about in terms of flavourful ingredients, while some people might think an ongoing parade of flavour is "boring".

So, a question is what "tasty" (and thus "non-bland") were available before Asian (and American) ingredients came onto the scene in European food. Since talk of spices and exotic ingredients is usually in the context of Medieval cuisine, or later, I will assume that we don't need to consider major foods that were Asian domesticates (e.g., wheat, barley, sheep, etc.) as "Asian".

There were many very flavourful European foods that were available: fermented fish sauces/pastes, as discussed in https://www.reddit.com/r/AskHistorians/comments/cv8ij1/is_there_a_historical_connection_between_roman/ and various fermented animal products such as tasty cheeses and fermented meat products such as hams and salamis. Apart from these unami-bombs, there are also onions, garlic, and many, many herbs and spices (many of which are prehistoric Asian domesticates, but some of which may well be European in origin, e.g., coriander/cilantro).

These "tasty" ingredients weren't always desired! For example, Liutprand of Cremona, Italian ambassador to the Byzantines c. 10th century, complained about these, while writing more positively about "a fat goat, of which he himself had partaken, deliciously stuffed with garlic, onions and leeks; steeped in fish sauce: a dish which I could have wished just then to be upon your table, so that you who do not believe the delicacies of the sacred emperor to be desirable, should at length become believers at this sight!" also complained rather more harshly about a "disgusting and foul meal, which was washed down with oil after the manner of drunkards, and moistened also with a certain and other exceedingly bad fish liquor".

If we look at ancient Roman recipes, the main imported ingredient is pepper (e.g., search for recipes from Apicius). Taking away pepper, there is plenty left that is tasty. If we move away from the places and times of fish sauce, there is plenty of tasty variety left: smoked and fermented meats, and cheeses. An immense variety of herbs, strongly flavoured greens (who ever said that foods like dandelion, rocket, endives, etc. are bland/boring?). Exceedingly tasty (and not necessarily in a good sense - YMMV!) fermented fish products such as surströmming, rakfisk, and so on.

There are some European cuisines that have well-established reputations for being bland and/or boring. Notably, most of them have managed to maintain these reputations in the face of the modern availability of Asian and American ingredients. "Boring" and "bland" do not mean that nothing better (for personal definitions of "better") weren't possible.