How can I fix my tasteless baked flounder?
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.