Cook macaroni according to package directions. Drain and transfer to a large bowl. Let macaroni cool for a minute or two, then stir in sour cream; set aside.
Meanwhile, melt margarine in a large saucepan over medium heat. Transfer 1/2 tablespoon of the margarine to a small bowl and add bread crumbs and Parmesan cheese. Mix with a fork until mixture is crumbly; set aside.
Add Cheddar and mustard to remaining 2 tablespoons melted margarine. Set pan over medium heat and cook 2 minutes, until cheese melts, stirring constantly. Add sour-cream-macaroni mixture and peas and cook 1 minute to heat through, stirring constantly to coat macaroni with melted cheese. Transfer mixture to individual plates and top with bread-crumb mixture.
Serve with a salad of mixed baby greens and balsamic vinegar and olive oil.