First, make the sauce: heat oil in a large saucepan and add onion, celery, capsicum and garlic. Cook until onion is translucent.
Pour the tin of tomatoes into a sieve and push through with the back of a wooden spoon, making a puree. Add the puree to the saucepan and discard remaining pulp.
Add parsley, basil, sugar and tomato paste and mix well. Dissolve the stock cube in the boiling water and add to the saucepan. Bring to a simmer and simmer with the lid on 30 minutes. Remove lid and simmer until sauce is thick. Season with salt and pepper.
Dip the chicken into beaten egg and toss in breadcrumbs to coat. Place on a plate and chill until needed (sauce and chicken can be prepared in advance).
Preheat the oven to 180 degrees C.
In a nonstick frying pan, heat olive oil and butter. Pan fry the chicken until golden then place in an ovenproof dish. Sprinkle over the mozzarella then spoon over as much sauce as you desire (the remainder freezes well). Sprinkle over Parmesan. Cover with foil.
Bake 15-20 minutes, then remove foil and bake an additional 10 minutes. Serve.