Start off by preparing the filling. Peel and then finely chop the onions. Melt the butter in a large saucepan and add in the onions. Sauté until browning slightly.

Add in the flour to the onions and stir well. Continue to cook while stirring for a few minutes until the flour has browned slightly. Next mix together the water and milk and add into the pan with the onions, bit by bit, stirring well between each addition, until you have quite a thick, but smooth sauce.

Add in the potatoes and sweetcorn. Slice the fish into good sized chunks and add in as well, before snapping the ends off the asparagus spears. Discard the ends and slice the heads up into chunks and adding to the pot.

Reduce the heat a little and allow to simmer gently until the potato chunks are cooked through.

While the filling is cooking, prepare the topping. Boil the potatoes and then finely mash them with the butter and milk. Stir in the grated carrots and mozzarella cheese.

Once the filling is ready, transfer to a large, deep, oven-proof dish and top with the mashed-potato topping - smoothing out with a fork or similar implement to create a nice-looking surface.

Preheat your oven to about 170 degrees Celsius. Place the pie into the oven and bake for about 30 minutes. For the last 5 minutes of cooking, turn on your grill (broiler) to allow the topping to crisp up.

Serve immediately.