Fish Pie I

Preheat the oven to 350°F (175°C).

Heat the butter or margarine in a large pan over medium heat, then add the chopped onions and sauté until softened and translucent, about 5 minutes.

Stir in the dry mustard, salt, and pepper into the onions, mixing well to combine the spices.

Add the flaked fish and tomato sauce to the pan, stirring gently to combine without breaking up the fish further. Cook for 3-4 minutes until heated through.

Transfer the fish mixture to a buttered baking dish, spreading it evenly across the bottom.

In a bowl, combine the beaten eggs with the water, whisking until well blended.

Mix the mashed potatoes with the egg mixture until the potatoes are light and creamy, stirring gently to avoid overworking.

Spread the potato-egg mixture evenly over the fish layer in the baking dish, smoothing the top with the back of a spoon.

Bake in the preheated oven for 20-25 minutes until the potato topping is golden brown and the pie is heated through.

Remove the fish pie from the oven and let it rest for 2-3 minutes before serving.