Place uncooked macaroni in glass dish with 550ml boiling water.
Cover, place on oven base and cook on High Microwave for 6 mins or until soft.
Drain in colander.
Place butter and onion in a jug and cook on High Microwave for 5 mins or until soft. Stir halfway through cooking.
Stir in flour and cook on High Microwave for 30 seconds. Gradually add milk, stir well and season.
Cook on High Microwave for 9 mins until sauce is thick and bubbling. Stir twice during cooking.
Add mustard and 100g grated cheese. Place macaroni and sauce in dish and stir well. Sprinkle with breadcrumbs and grated cheese.
Place dish in glass shelf in the lower shelf position and cook on Convection 230 degrees C + Grill 3 + Low Microwave for 12 mins or until cheese starts to melt. For a bit of added crunch give it an extra 2 minutes under the grill.