Place in a non-stick saucepan the onion with the garlic and stock. Gently cook for 4 to 5 minutes, stirring frequently. Remove all visible fat from the bacon (or buy the medallions). Cut into strips. Add to the pan and cook on high until the bacon has cooked through.
In a small bowl whisk the quark or fromage frais, eggs, Parmesan cheese, parsley and seasoning.
Bring a large pan of salted water to the boil and cook the pasta until tender with a bite at the centre. Drain and return to the saucepan on a very low heat. Add the bacon mixture and then the egg mixture. Stir and cook for a further 3 minutes.
Instead of the bacon you can use peas, green beans and asparagus to make it a primavera sauce. Also try it with smoked salmon and peas. Very versatile sauce and delicious.