What to do with tasteless fish?

Spice them up with a Mexican mix of 'taco' type seasoning and make fish tacos.

like another poster, i'd vote for fish soup. i've been making both chicken and beef (short ribs) broth and i always have either raw or oven cooked fish (ling cod or wild alaska salmon) on hand. either goes into the broths with fresh separately cooked veggies for really healthy soups. doesn't matter if flavorful or mild fish.

If you have access to fish sauce, make ca kho to (Vietnamese braised fish):
http://www.theravenouscouple.com/2009/08/ca-kho-to-vietnamese-braised-fish-in-clay-pot.html

One of my childhood favorites, the fish steaks are braised in an intense caramel-y sauce. So wonderful with plain white rice.

Or poach in sauce: https://chinesegrandma.com/2015/05/soy-sauce-poached-fish/

If you can find a recipe where the fish is braised or poached in a highly seasoned liquid, it should help with bland fish. Good luck!

Make a fish stew or soup that depends on other elements for its flavor. Two that jump for late summer hot weather are a corn chowder (add the fish to any recipe that you like) and various south Indian fish curries. For example:
https://www.thespruceeats.com/south-indian-fish-curry-1957456
PS Also let your fishmonger know of the bland taste...feedback always helps...maybe something is going on in that patch of water where they get their fish.

Two options come to mind: If you have access to a smoker, smoked trout is wonderful or you could make a flavorful stuffing (maybe with crab) and do a whole roast.