Cook macaroni as directed. Saute chopped onion in saute pan with 1 Tbsp. of butter. When onion is translucent, add Miracle Whip and milk and blend. Add Velveeta and stir till smooth. Mix macaroni, tomatoes, chicken, and peas together.
Pour into casserole dish. Pour sauce over macaroni. Melt 1 Tbsp. of butter and mix with bread crumbs. Add parmesan cheese to bread crumbs and spread over casserole.