This casserole makes an excellent one-dish fish meal – and the spinach adds nice color.
In a large skillet, saute fish, onion, celery, salt, and pepper in oil for about 10 minutes. Set aside to cool slightly. Cook spinach and drain, then squeeze out all water. Set aside to cool. Mix mashed potatoes, sauteed fish, and beaten egg together. In a buttered casserole dish, place a layer of the fish mixture, then a layer of spinach. Sprinkle with half the grated cheese. Cover with the rest of fish mixture, then the spinach, and sprinkle the remaining cheese over top. Bake at 350 degrees F for 25 to 30 minutes until just heated and the cheese melts. Serve with a white or fish sauce.