Heat oven to 220C/200C fan/gas 7. Make the tomato sauce in a medium pan by heating the oil and frying the garlic for 2-3 mins until soft, then adding the spices and frying for 1 min more. Pour in the tomatoes and sugar and simmer with half a tin of water for 20 mins, until thickened. Season.
Meanwhile, scoop out and discard some of the flesh of the courgettes and lay them in 1 large or 2 small roasting trays, drizzle with the oil and bake for 15 mins until golden and softened slightly.
For the stuffing, mix the lamb with the spices and some salt. Try not to squash it together too much, instead lightly mix it with your hands until just combined. When the courgettes are ready, pile the lamb into the cavities and drizzle on the sauce. Bake for 15-20 mins until the sauce is bubbling. Top with the coriander and serve.