Step 2

Melt 50g butter in a medium saucepan over medium heat. Add the onion and cook for 5 minutes, stirring occasionally. Add the mushrooms and cook until both are almost softened.

Step 3

Stir in the flour to form a thick paste (roux). Crumble in the stock cube and stir well. Cook the roux over low heat, stirring continuously for 2 minutes to cook out the floury taste.

Step 4

Remove from the heat. Gradually add 500ml of water, a little at a time, stirring constantly to keep the sauce smooth. Season with black pepper, a pinch of nutmeg, and mustard powder.

Step 5

Return to medium heat and bring slowly to a boil, stirring continuously until the sauce thickens. Reduce to very low heat. Stir in the cooked chicken and sweetcorn.

Step 6

Grease a medium ovenproof dish with butter and pour in the filling. Lay the potato slices on top in overlapping layers, like a pie crust.

Step 7

Brush the potato slices with melted butter and bake for about 35 minutes, until the potatoes are cooked through and golden brown.