Add a little olive oil to a saucepan and heat. Once heated add the onion and garlic then cook and stir over a medium heat until softened. Add the beef and cook for 10 minutes or until lightly browned all over.
Once the meat is browned add the sweetcorn, tomatoes, mixed herbs, salt and pepper to your liking. Reduce heat, cover and simmer, stirring occasionally for about 25 to 30 minutes.
Bring a saucepan of water to the boil over a high heat. Add macaroni and a bit of salt and cook according to the instructions on the packet or until al dente. Drain.
Melt margarine in a saucepan over a medium heat and add flour then stir until all combined. Remove the saucepan from the heat and stir in the milk a little at a time. Once all the milk has been added return to heat and bring to the boil, stirring constantly. Once it starts boiling, lower the heat and stir for 5 minutes until the sauce has thickened.
Remove saucepan from heat and stir in the mustard and 150g grated cheese, stirring well until melted. Add the drained macaroni into the cheese sauce, stirring well to mix.
Preheat the grill to medium high.
Place the beef mixture onto a baking dish, cover with the macaroni mixture, then sprinkle the remaining grated cheese over and place under the pre-heated grill for 4 to 5 minutes or until golden brown. Remove from the heat and serve immediately.