Layer the spinach in the bottom of a 2 litre baking dish. Lay the rockfish atop the spinach. Scatter the tomatoes around the fish. Pour the stock 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 aluminium foil.
Bake in preheated oven until the fish flakes easily, 20 to 25 minutes.