I may have reinvented the wheel; it wouldn't be the first time. Since last summer, I've gotten free local fish fillets every other week, in exchange for manning one of the distribution locations for a community-supported fishery. This latest 8-wk cycle, they've asked me to do it weekly, so I need a few more ways of cooking my bounty (don't want raw or deep-fried).
Today I soaked some dried vegetable mix (carrot, onion, bell pepper, celery) in water for several hours till they were well rehydrated. Stirred in a generous dollop of bottled thousand island dressing, a squeeze of sriracha, and thinly-julienned leek greens. Slathered this onto the haddock and baked. This is quite delicious and the idea of using salad dressings, whether purchased or homemade, does open up a lot of possibilities. You could of course use fresh vegetables, which would involve more hands-on prep, and bake a little longer to compensate for the additional water in the fresh mixture.
It should also work to use just the dressing (as long as it's a thickish one) before baking or broiling. My crummy camera made it look too dark, which it isn't. Spooned out a bit on the lower portion of the gratin dish, to show that the sauce is reddish brown.