In small bowl, whisk together sugar, mustard, and salt until well blended. Whisk in oil, a small amount at a time. Whisk in vinegar, a small amount at a time. Set dressing aside.
Cook pasta according to package directions. Drain, rinse with cold water, and drain again. Transfer to large bowl.
Add ham, broccoli, and onion to the pasta. Toss lightly with dressing to coat evenly. Cover, and refrigerate at least 2 hours. Garnish with tomatoes.
