Place the tomatoes cut side down on a rimmed baking sheet and broil until the skins begin to char and burst, 10 to 12 minutes. Remove from the oven. When cool enough to handle, remove the charred skins.
Meanwhile, heat a large saucepan over medium heat. Add the butter and then the onion. Cook and stir until tender but not brown, 3 to 5 minutes. Add the tomatoes and simmer until the tomatoes have cooked down, about 20 minutes. Season with the salt, pepper, and sugar and add the basil and oregano. Serve immediately, or remove from the heat and cool completely before storing in the refrigerator or freezer.