Preheat oven to 200°C. Clean the squash, cut in half at its equator and scrape out the seeds.
Place the two halves face down in about 2 cm of water in a small roasting pan then put in the oven for 25 or 30 minutes.
Meanwhile, peel & finely chop the garlic and onion. Heat the oil in a medium-sized heavy-based casserole dish and add the garlic, onion, lamb. Stir over a medium heat for 5 minutes to brown the meat, adding the herbs and seasoning.
Pour in the tomatoes and stock. Bring to the boil, then turn down to simmer for about 15 minutes, allowing the liquid to reduce so it's not too runny.
Place in a suitably-sized ovenproof dish. Take the squash from the oven and spoon the flesh over the meat then smooth it down a bit with a knife.
Sprinkle some grated cheese over the top, then pop under a hot grill until nicely browned.