Preheat the oven to 220C/Gas 7/fan oven 200C. Peel and thickly slice the potatoes, then boil in salted water for 8-10 minutes, until just tender. Drain.
Chop onion finely and fry in 25g/1oz butter and a drizzle of olive oil for 5 minutes. Snip bacon into pieces with scissors and add to the pan; stir well and cook for a further 5 minutes, until the onion and bacon are lightly coloured.
Chop cheese into chunks, rind and all. Layer half the potatoes in a 1.5 litre/2¾ pint buttered ovenproof dish and scatter over half the onion, bacon and cheese. Lightly season with salt and pepper. Repeat layers, then pour cream evenly over the top and bake for 10-12 minutes until golden. Rest for 5 minutes and then serve with a salad.