Position a rack in the center of the oven and set the broiler to high.
Place the tomatoes on a sheet pan and broil until the skin is charred and peeling, about 10 minutes. Broilers can be incredibly fickle, so start checking for char marks after 5 minutes. Flip and char the other side, until the tomatoes are soft and begin to release some of their juices, 5 to 7 minutes. Remove and set aside.
Once the tomatoes are cool enough to handle, coarsely chop them into small pieces. In a medium bowl, combine the tomatoes, their juices, the vinegar and garlic. Season with the pepper flakes, ½ teaspoon salt, and ¼ teaspoon black pepper. Add 3 tablespoons olive oil and stir. Taste and adjust with more salt, black pepper, and oil as needed.