1 Season the fillet with the dill, garlic powder, lemon pepper, onion powder, salt, and pepper. 2  Place the lemon, onion, and butter on the rockfish. 3  Cover the entire dish with aluminum foil. 4  Bake in preheated oven until the fish flakes easily, 20 to 25 minutes. Layer the spinach in the bottom of a 2 quart baking dish. 2  Lay the rockfish atop the spinach. 3  Scatter the tomatoes around the fish. 4  Pour the broth into the dish. 5  Season the fillet with the dill, garlic powder, lemon pepper, onion powder, salt, and pepper.