In a medium saucepan, combine the tomatoes with the olive oil, rosemary, garlic and a pinch of salt. Cook over moderately low heat, without stirring, until the tomatoes are tender but still intact, about 10 minutes. Give the tomatoes a gentle push if necessary, to keep them submerged. Using a slotted spoon and allowing as much of the oil to drain back into the saucepan as possible, transfer the tomatoes to a plate. Reserve 3 tablespoons of the oil; strain and refrigerate the rest for later use.