1. Peel and chop the potatoes, butternut squash and carrots. Boil them together in salted water until completely soft.
2. Drain them really well, then leave them in the hot pan for a couple of minutes so the steam can escape. Mash with a good knob of butte. Don't add much milk or cream - the squash already adds moisture, so you want the mash fairly firm.
3. Cut the bacon into strips and fry over a medium heat until it starts to colour.
4. Add the sliced onion and leek. Turn the heat down and cook for about 8-10 minutes until soft.
5. Pour in the 150ml of cream and let it gently bubble for a couple of minutes.
6. Stir in around 100g of double Gloucester and 50g of cheddar until melted. Add plenty of black pepper and, if you like, a little garlic granules or mustard.
7. Spoon the creamy bacon and leek mixture into an ovenproof dish.
8. Spread the mash over the top.
9. Sprinkle over the remaining Gloucester and cheddar.
10. Bake at 200C/180C fan for around 25-30 minutes, until bubbling around the edges and golden on top.