Heat oil in a small saucepan on medium, and add onion and garlic and cook uncovered, stirring occasionally, for 6-7 minutes or until onion has softened.
Add tomatoes and bring to a simmer, and simmer uncovered for 15-20 minutes or until sauce is thick.
Meanwhile, prehat oven to 180C (fan forced) and line a large baking tray with baking paper.
Place a chicken fillet between 2 pieces of greaseproof paper (or plastic wrap) and using flat side of a mallet (or small frypan/skillet also works well) and pound until 1.5cm thick and place on a a plate, and repeat with remaining fillets.
Put flour on a plate.
Whisk together milk and egg whites in a shallow dish.
Place breadcrumbs on a shallow plate.
Coat chicken in flour, then egg mix and finally breadcrumbs and place on lined tray, repeat with other 3 slices of chicken.
Spray the chicken with cooking spray and bake for 8-10 minutes or until cooked.
Preheat grill/broiler to medium.
Transfer cooked chicken to a shallow ovenproof dish and place a slice of ham on each piece of chicken.
Stir sugar and basil into tomato sauce and spoon over hammed top chicken and top with cheese.
Grill/broil for 4-5 minutes or until cheese melts and turns golden brown.
Serve chicken with steamed vegetables (in a steamer would take about 10 minutes for a medium soft texture, about 8 for a al dente taste or 6 for crisp).