Heat oven to 220C/200C fan/gas 7. Place the onion, courgettes and red pepper in a baking dish, large enough to fit them in a single layer. Drizzle over 2 tbsp of the olive oil and season. Cook in the oven for 20 mins.
Meanwhile, heat the remaining olive oil in a frying pan and cook the sausages for about 5 mins until lightly browned on all sides.
Stir the garlic and chopped tomatoes into the veg, season again, then place the sausages on top and return to the oven to cook for a further 20 mins until the vegetables are tender.