Place sweet potato in microwaveable bowl with 60 mL (1/4 cup) of water. Cover and microwave on HIGH for 4 to 6 minutes or until very soft. Drain and mash with potato masher until smooth; set aside.
Meanwhile, in pot of boiling water, cook macaroni for about 8 minutes or until tender but firm. Drain well and return to pot.
In saucepan, melt margarine over medium heat and stir in flour. Cook, stirring for 1 minute or until thickened. Slowly whisk in milk and cook, whisking occasionally for about 8 minutes or until starting to bubble around the edge. Whisk in cheese and mustard until smooth. Whisk in sweet potato and add peas. Pour into macaroni mixture and stir until well combined.
Breadcrumb topping: In small bowl, combine breadcrumbs and margarine.
Scrape macaroni mixture into 2 L (8 inch) glass casserole dish and sprinkle with breadcrumb topping. Bake in 190 C (375 F) oven for about 15 minutes or until golden and bubbly.