Cook and drain pasta as directed on package, omitting salt. Rinse with cold water; drain well.
Meanwhile, in 10-inch nonstick skillet, cook bacon over medium heat about 4 minutes, turning occasionally, until crisp. Drain on paper towels. Crumble into small pieces or cut into thirds.
In large bowl, mix pasta, onion and tomatoes; toss gently with dressing to coat. Place lettuce leaf on each of 6 salad plates; divide pasta mixture evenly along plates. Sprinkle each with bacon.