Cook pasta according to package directions, omitting oil or salt in process. Drain; rinse with cold water; drain again.
In a large bowl, toss together drained pasta, carrot, cucumber, sweet pepper and green onions. Add salad dressing, toss to coat. Gently stir in tuna. Cover and chill in refrigerator for 2–6 hours.
Arrange mixed greens or lettuce on plates. Spoon tuna mixture over greens.