Fry the onion and courgettes in 2 tbsp of olive oil for 8 minutes, until golden. Add the tomatoes, lemon juice, basil and season. Cook until the tomatoes are heated through, transfer to a dish to keep warm.
Slice each chicken fillet into 2 thin flat pieces. Dip into the egg white and coat both sides in parmesan.
Wipe out the pan, add 1 tbsp oil and cook the chicken for 2-3 minutes on each side over a high heat until crisp. Serve with the courgettes and lemon wedges.