- Cook macaroni in boiling salted water until tender, approximately 15 minutes.
- Drain and rinse under running water.
- Chop bacon and peel and chop onion finely.
- Melt butter in a pan, and saute onion and bacon until bacon is crisp. Remove from pan.
- Stir flour and mustard into butter remaining in pan. Cook 1 minute.
- Remove from heat and gradually add milk.
- Return to heat and stir until sauce boils and thickens. Season with salt and pepper.
- Grate the cheese and stir in three-quarters of the cheese until it melts.
- Add bacon, onion and macaroni.
- Spoon into a well greased ovenproof dish.
- Slice tomatoes and place slices on top of mararoni mixture.
- Combine extra melted butter and breadcrumbs. Sprinkle over the top of the macaroni and tomato.
- Top with remaining grated cheese.
- Bake in a moderate oven for 25 to 30 minutes until we11 browned.