Bring the salted water for the spaghetti to a boil in a large saucepan. Peel and finely chop the onion. Pat the meat dry, flatten it carefully, lightly season with salt, pepper and dust with flour. Cook the pasta in water for approx. 8-10 minutes until al dente. Whisk the eggs with the parmesan, heat the butter and 1 tablespoon oil in a pan over a medium temperature.
Turn the schnitzel thoroughly in the egg-parmesan mixture and fry in two portions in the fat at a medium temperature on each side for approx. 2-3 min. Add a little more fat if necessary.
Take the schnitzel out of the pan, keep warm (in the preheated oven at 100 degrees). Pour the remaining oil into the pan, sauté the onion, add the tomatoes, heat, salt, pepper and season with oregano.
Serve the schnitzel with the tomato sauce and the spaghetti.