I just roasted some beets in the oven (as usual) but they turned out really bitter. Any idea why or what can I do to save them?
Roasting caramelizes the natural sugars in vegetables and, eventually, burns them. Roasted anything that turns out bitter is almost always the symptom of overcooking. Sometimes it also means that your ingredients were spoiled or "off" in the first place.
As for saving them - there's not much you can do in either case, other than trying to mask it with some other flavours. Beetroot pairs well with cranberry and other tart fruits, which would probably mask the bitter taste; cream, sweetened yogurt or sour cream would also be a good bet. Otherwise, I can't really think of anything else to do with them; overcooked food usually ends up in the trash for me.
Salt is quite effective at neutralizing bitter flavors, which is why some folks experiment with using low levels of it in coffee. For beets, go ahead and salt them a bit more aggressively than you normally would and see if that salvages the flavor for you. Here are a couple of references:
I found some beautiful yellow, orange and pink beets in a market in Tucson.I boiled them, the skins slipped right off. When I tasted some I was so disappointed at the strong bitter after taste. So I let them cool, sliced and fried them it a little coconut oil. It took away the bitter taste and was delicious. I don't know the consistency of a roasted beet but it you can slice it you might save them by frying.