2. Place a roasting tray on the hob over a medium-high heat. Add the onions, garlic and cayenne pepper, drizzle with enough olive oil to coat, and cook for 3–4 minutes. Add the tomatoes cut side down, season with a pinch of salt, pepper and the sugar, drizzle over the balsamic vinegar and cook over a high heat for 3–4 minutes, until the tomatoes begin to caramelise. Stir well, then transfer to the oven for 20–25 minutes.