Cook macaroni according to package directions. Drain and pour into a greased 9 x 5 inch (2L) casserole.
Melt butter in saucepan and saute onion until soft. Add flour; stirring for 1 minute. Slowly add milk, season with salt and pepper. Cook, stirring over medium heat until slightly thickened. Whisk in mustard, add cheese and stir over low heat until melted. Pour over macaroni, add tomatoes mixing well.