Cheese, Onion and Potato Pie

[sjj1805]

Serves 2 generously

Ingredients
750g potatoes, peeled and quartered
40g butter
1 large onion, thinly sliced
100ml milk
200g mature Cheddar, Double Gloucester or Red Leicester cheese, grated (I used cheddar)
1 tsp wholegrain or English mustard
Sea salt and freshly ground black pepper
2-3 tomatoes, sliced

Preheat the oven to 200oC/fan 180oC/gas 6. Cook the potatoes in boiling salted water for 15 to 20 minutes until tender.
While the potatoes are cooking, melt the butter in a pan and gently sauté the onion until lightly golden and sweet. This will take about 10mins. Add the milk to the pan and heat until almost boiling.
Drain the potatoes and return to the pan. Mash really well with a potato masher, and then beat in the milk and onion, three-quarters of the cheese, the mustard and some seasoning.
Transfer to a buttered oven proof dish, scatter over the remaining cheese and lay the tomato slices on top and season again if you wish.
Bake for 20 to 25 mins until the top is bubbling and lightly golden.