To blanche the tomatoes: Score tomatoes with a cross on the bottom. Place the tomatoes in boiling water for 2-3 minutes until the skin starts to peel slightly. Remove from boiling water and remove skins. Cut into quarters and remove seeds. Set aside.
In a food processor combine prepared tomatoes, garlic, basil, olive oil, salt and pepper to taste. Puree until smooth. Add balsamic vinegar to taste.
Strain excess liquid if necessary. Pesto will keep for 3 days in the refrigerator. Yields 1½ cups.