Bring large pot of salted water to boil over high heat. Add pasta and cook to al dente. Drain, rinse, and set aside to cool for 5-10 minutes.
When pasta has cooled, using large mixing bowl, combine pasta with diced vegetables, mozzarella, mayonnaise and Dijon mustard. Mix well. Season with salt and pepper to taste.
When ready to serve, lay out lettuce leaves on large plate or platter. Place 1 to 2 tablespoons of the macaroni and cheese salad inside each leaf and serve immediately.