2 onions, chopped
6 large potatoes, cubed
1 medium cauliflower or romanesco, broken into florets
1 tsp mustard
All the leftover cheese you have in your fridge, grated
A splash of milk
1) Fry the onions in the butter in a large pan, then add the potatoes and cauliflower. Cover with salted water and boil until tender. Meanwhile pre-heat the oven to 180C/350F/gas mark 4.
2) Drain and mash the cooked vegetables, then add the mustard. While the mixture is still hot stir in most of the cheese to melt it, keeping a little cheese to sprinkle on the top of the pie.
3) Beat in the egg with the milk, then season.
4) Transfer the mixture to an ovenproof dish, then rough the top with a fork. Sprinkle the rest of the cheese over the top.
5) Bake until the top is brown and crispy.