Place lamb, onion, bouquet garni,
Turn the oven to 220°C/450°F/Gas Mark 6. Drain cooking liquid from the pan, place everything into a roasting dish and bake 15 minutes. Meanwhile, place some finely sliced garlic and diced red onion in each capsicum half. Add 2-3 tomato halves, cut-side up, to each one.
Take lamb from the oven, drain off fat, discard vegetables and turn lamb over. Place capsicums in the dish and drizzle everything with olive oil and vinegar. Season with salt and pepper. Turn oven to 160°C/325°F/Gas Mark 3 and cook the lamb to whatever stage you like (about 15 minutes for medium). You may need to cook the capsicums slightly longer. Rest lamb for at least 30 minutes before carving.