Cut the ham into thin strips. Peel and halve the onion and cut into thin slices. Grate cheese. Put aside approx. 1 tbsp. cheese for sprinkling. Prepare pasta in boiling salted water according to package instructions
Heat the oil in a large frying pan. Fry the ham until crisp. Take it out and let it drip off on kitchen paper. Sauté the onion in the pan for 2-3 minutes. Add crème fraîche and stock and simmer for about 5 minutes. Add cheese, season with salt and pepper
Drain the pasta. Mix cheese sauce, approx. 3/4 of the ham and noodles in the pot, bring to the boil. Serve immediately sprinkled with the remaining ham and cheese