Preheat the oven to 200°C, 400°F, Gas 6.
Blanch potatoes for 5 minutes.
Place the lamb in a large non-stick roasting tin, scatter around the carrots, squash, potatoes and red onions. Drizzle over the oil and season all well.
Add a small glass of wine and a splash of water.
Place the roasting tin in the oven for 10 minutes and then turn the oven down to 180°C.
After another 30 minutes add the garlic and rosemary sprigs to the pan and scatter the fennel seeds and lemon over the lamb.
Continue roasting until the vegetables are soft and beginning to caramelise and the lamb is cooked to your liking (approx 20-30 minutes more for medium).
Add a splash more wine or water to the pan if it is looking very dry. Once out of the oven cover with foil and rest for 15 minutes before carving.