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