Heat the oil and butter in a pan . Add in the sliced onion. Cook gently until coloured

Mash the potatoes with a good knob of butter and plenty of salt and pepper

Stir through all but a handful of cheese

Add in the onions stir to mix well

Grease an ovenproof dish. To the cheesy potato mix into the dish . Sprinkle over the remaining cheese . Top with sliced tomato

Bake in a 200c oven for about 25 minutes until golden and crusty

Serve

You can even mage this dish with instant or frozen mashed potatoes