Preparation Prepare a grill or grill pan for medium-high heat. Slice 2 tomatoes in half and brush each half with oil. Brush onion with oil. Transfer onions and tomatoes halves to hot grill and cook, turning occasionally, until slightly charred and softened, about 10 minutes. Let cool slightly. Remove seeds from remaining 2 tomatoes and cut into 1/4" cubes. Transfer to a medium bowl. Add jalapeño and salt and toss to combine. Finely chop cooled onions and tomatoes, then add to raw tomato mixture and toss to combine. Do Ahead Salsa can be made 1 day ahead. Transfer to an airtight container and chill.