Heat the olive oil in a large skillet over medium heat, and cook and stir the onion for about 5 minutes, until translucent. Add the tomato wedges and mushrooms, and simmer, stirring occasionally, for about 20 minutes, until the tomatoes and mushrooms are cooked through and the sauce is reduced and thickened.
Sprinkle on the basil, salt and pepper, and stir to combine.
