Place tomatoes in a small saucepan and season with the salt. Add the rosemary sprig and garlic and enough olive oil to barely cover the tops of the tomatoes. Bring oil to a very low simmer and cook for about 1 hour. Remove tomatoes from the oil with a slotted spoon and place in a food processor or blender. Process until smooth, 1 to 2 minutes.
Yield: about 1/2 cup