I fixed flounder fillets in a bed of yellow onions and mushrooms in a 425°F oven -- as the recipe says -- and the fish does not taste like anything. The Hollandaise sauce did not help either. Is there anything that I can do to improve the flavor or is trashing it the only option?
Most fish is pretty mild, and flounder is no exception. You shouldn't expect it to have a terribly strong flavor of its own; that's why you've made a recipe that cooks it with onions and mushrooms and adds Hollandaise sauce. If you eat it with all of that, it'll taste like mild fish with mushrooms, onions, and Hollandaise - hardly tasteless. If you think even with all the other stuff there's not enough flavor, you might want to add a bit of salt; salt increases our perception of flavors, and if you're used to it, undersalted food will taste bland. But otherwise, it sounds like you just don't like fish, mushrooms, onions, or Hollandaise sauce and you should make something different next time.
And maybe this is subjective, but I can't imagine that this could possibly be so tasteless that you'd rather throw it away than eat it, accepting that it's not your favorite food.
A decent oven recipe I've used has been Anthony Bourdain's, from Kitchen Confidential.
You essentially rub some salt (your choice), make a bed (and blanket) of sliced lemon and garlic, cover and seal the whole thing in foil so the steam doesn't escape (if you don't like foil, use a Dutch oven).
It only takes about 20 minutes. Anthony says it's a $40 dish (in jest). Done right, this is a nice and tasty dish.
Flounder is my favorite fish, flakey and mild. Imagine putting onions and mushrooms with let’s say Fresh Mozzarella… to harsh, you lose the wonderful mild flavor. IMO flounder is best when kept simple.
2 to 3 tbsp. of EVOO coat the fillets and center in shallow small pan. Dice a tomato and surround the flounder. S & P with ½ tsp. of chopped parsley (optional). Sprinkle top with Paprika (optional but delicious). 3 Tbsp. of dry white wine or dry vermouth. Lightly cover with foil and cook in oven 400 deg. F for 10 mins, remove foil and cook for another 5 mins. A squeeze of lemon with it comes out and that’s it. Generally there will be a lot juice in the pan from the fish and tomatoes. Make enough rice or pasta. My husband who is a big fishy fish eater loves this dish. I get a big yumooo from him when he is finished.
Variations: Add some diced red pepper to the tomato. Press 1 garlic clove add to the tomatoes and add some shrimp. Add chopped spinach to the tomatoes. Instead of rice or pasta, add “toasted” not just dried bread seasoned bread crumbs the last 5 mins. I thinly sliced 3 Brussel sprouts and sprinkled on top with a dollop of butter. Whatever you choose, don’t add too much and overwhelm the fish. Simplicity is key.