Fish Pie

Cut 700 g potatoes into even pieces and boil them for 15 to 20 minutes, until a skewer reaches the center easily. Drain thoroughly and let the steam escape before mashing with some of the butter and milk. Keep the mash soft enough to spread but not runny, since it must form a stable top layer. In a separate pan, cook 120 g finely chopped onion over medium-low heat for 4 to 5 minutes until translucent. Add the remaining butter and 2 tablespoons all-purpose flour, stirring for about 1 minute to remove the raw flour smell. Pour in 350 ml milk in several additions and whisk after each one so the roux loosens smoothly without lumps. Reduce the heat when the sauce lightly coats the back of a spoon. Add 500 g white fish cut into bite-size pieces and 80 g peas, then fold gently over low heat for only 3 minutes. The fish will continue cooking in the oven, so it should begin turning opaque without breaking apart at this stage. Spread the filling evenly in a baking dish and cover it with mashed potato all the way to the edges. Draw ridges across the surface with a fork and scatter 60 g cheddar over them; the raised areas give the topping more places to brown. Bake in a preheated 200 C oven for about 20 minutes. Once the sauce bubbles around the sides and the potato is golden, rest the pie for 5 minutes so the filling settles and each serving holds both layers.