Slice the tomatoes in halves. Place them on baking tray with the flesh facing upwards. Drizzle with the olive oil and sprinkle with salt and pepper. Place the tray in the oven and slow roast the tomatoes at 150 degrees Celsius for 35-40 minutes. Make sure not to roast the tomatoes at higher temperature otherwise they will end up looking like a tomato sauce, you want to cook them slowly at a low temperatures they will keep their shape, almost like sundried tomatoes.
In the meantime make the tomato sauce. Add the oil to a large pan. Once hot add in the chopped onion. Let the onion sauté for about 5 minutes until it starts to caramelise and going slightly brown. Add in the crushed garlic and keep cooking for another minute.