Preheat oven to 375ºF. Remove packaging from thawed Mahi Mahi portions and pat dry with paper towels.
Combine flour, salt and white pepper in a shallow container.
Beat eggs in a second shallow container.
Melt butter in a nonstick skillet. When melted, add breadcrumbs. Stir constantly over medium high heat for 3 to 4 minutes until breadcrumbs begin to darken. Transfer to a third shallow container.
Dip fish portion in flour, turning to coat evenly. Tap off excess flour. Place portion in beaten eggs; use fork to turn in eggs to coat evenly. Using fork, remove fish portion from eggs, drain slightly, and place in buttered, toasted breadcrumbs. Turn fish portion to coat evenly with buttered breadcrumbs, patting to press crumbs into portion but tapping off any excess.
Place portions on sheet pan sprayed with pan release. Spray each portion generously with pan release spray. Bake in preheated 375ºF. oven for 18 - 22 minutes (internal temperature of 145ºF.) Remove from oven.
Immediately turn broiler elements on “High”. Broil inside surfaces of hamburger buns or hoagie rolls until golden brown. Meanwhile, place a slice of American cheese on each mahi mahi portion.
When buns are golden brown, remove from broiler. Broil the cheese-topped fish until cheese begins to melt and drape over the fish portions. Remove fish from broiler, transfer to bottom of buns, and place a tablespoon (more or less to taste) of tartar sauce on each fish portion. Add bun top and serve.