Preheat the oven to 180˚C (160˚C for fan ovens) Gas Mark 4.
Put the onion, potatoes, peppers and courgettes onto a large roasting tin. Mix the crushed garlic with the oil, lemon juice and spices. Season with salt and black pepper and then whisk with a fork. Pour the marinade over the veg and cover with foil. Roast in the oven for 30 minutes.
Remove the foil, add the sausages and tomatoes, toss everything together and roast for a further 20-30 minutes until the sausages are browned.