Place macaroni in a microwave bowl, add a little olive oil and boiled water to cover. Microwave on HIGH for 5 minutes, stir then cook on HIGH for another 7 minutes or until pasta is al dente. Drain macaroni and set aside.
Preheat oven to 150 degrees C. While pasta is cooking Heat olive oil in a saucepan over low heat until warm, do not let it sizzle. Remove from heat and add flour, stirring quickly to dissolve any lumps. Add mustard and stir into mixture using a whisk. Return the pot to a low heat and gradually add milk, stirring consistently to avoid lumps. Add enough milk to make the amount of sauce desired for your dish, and until it is just about to your desired thickness. Add a bit over half the cheese and stir the cheese until melted. Season sauce with salt and pepper.
Cook chopped bacon and onion in a small microwave safe bowl in the microwave on HIGH for 5 minutes. Add to the pasta, and stir the cheese sauce through the pasta mixture until evenly distributed.
Transfer pasta into a large casserole dish, cover with the extra grated cheese, sliced tomatoes and sprinkle pepper over the top.
Bake in preheated oven at 150 degree C for approx 20 minutes. Serve with garlic bread.