Preheat oven to 180°C.
In a saucepan over medium heat, melt butter then add flour and cook, stirring, for 3 minutes. Add milk, stirring constantly, until thick and smooth. Add two-thirds of the cheese and Italian seasoning and stir until well combined. Remove pan from heat and set aside.
Spray a frying pan with cooking oil and place over medium-high heat. Add bacon and onion and cook, stirring regularly, for 10 minutes or until starting to colour. Add tomatoes and cook for a further 2 minutes and set aside.
In a mixing bowl, combine cooked macaroni, cheese sauce and bacon and onion mix. Transfer to an ovenproof dish and top with remaining cheese and breadcrumbs. Bake in oven for 15 minutes or until cheese and breadcrumbs are golden.