Rinse the chicken fillets under cold water, pat dry and plate. Salt, pepper and turn in flour. Whisk the egg on a plate and mix with the parmesan. Pull the schnitzel through the egg-parmesan mixture, turn the schnitzel in it and fry in clarified butter on both sides over a low heat until golden yellow. Drain on kitchen paper and keep warm.
Cook the spaghetti in boiling salted water according to the instructions on the packet until al dente.
For the tomato sauce: peel and finely dice the onion. Add the olive oil to the remaining frying fat in the pan and sweat the onions in it until translucent. Core the tomatoes, cut into small pieces and add to the onions. Add tomato paste and bring to the boil. Season to taste with salt, pepper and sugar.
For the pesto spaghetti:
Drain the spaghetti, drain and return to the saucepan. Add the pesto (bought or homemade) and a little more Parmesan and stir everything briefly.
Place the spaghetti with schnitzel and tomato sauce on a plate and garnish with basil leaves and lemon quarters.