Preheat the broiler. Set the tomatoes, cut side down, on a nonstick baking sheet and broil for 5 minutes. Remove the tomato skins and broil for another 2 minutes, or until just browned. Turn cut side up and broil for about 4 minutes, or until blackened. Transfer the tomatoes with any pan juices to a food processor and pulse until coarsely chopped. Add the shallots, balsamic vinegar and mustard. Season with salt and pepper and process until smooth. Transfer to a bowl.