You can either roast the tomatoes in the oven, in the pan or directly on the flame. To roast tomatoes in the oven, cut the tomatoes into large chunks and arrange them on a baking sheet along with chopped onion and garlic. Preheat the oven to 200 degrees and roast the tomatoes until they are slightly blackened. To roast the tomatoes in a pan on stove top, heat a little oil in the pan, and add the halved tomatoes in the pan. Cook until the skin of the tomatoes becomes a little charred. To roast the tomatoes directly on the stove top, using a tong, roast the tomatoes directly on the flame, until the skin of the tomatoes become black and charred.