Place the lettuce, tomatoes, and onion in a large mixing bowl. In a separate container, combine the salad oil and olive oil. Pour the oil mixture and lemon juice over the lettuce mixture. Add the garlic and salt. Toss until the lettuce is covered evenly with the oils, tomatoes, onions, garlic and salt. Do not over toss. Serve immediately or within 15 minutes to ensure lettuce remains crisp.