Preheat oven to 190 degrees celcius. Cut off and discard the top quarter of the garlic head and wrap the remainder in foil. Arrange the tomatoes (cut side up) on the foil-lined baking sheet and sprinkle lightly with salt. Add the red pepper and the foil wrapped garlic and roast for 1 hour until the tomatoes are just beginning to char, the garlic is completely tender and the pepper is cooked through with blackened skin. Transfer the pepper to a bowl, cover with cling film and leave to stand for 20 minutes. When cool enough to handle, peel and discard the core and seeds. Put the flesh in a food processor with the tomatoes. Add the lemon juice, olive oil ad balsamic vinegar and blend until smooth.