Rinse the chicken breast fillets, pat dry and cut into bite-sized pieces. The pieces go together with a tablespoon of oil and just as much butter in a large pan. Sear them well, season with salt, pepper and keep warm in the oven.
Peel the onion and chop finely. Heat another tablespoon of butter in the pan and fry the onion until translucent. Now add the mustard, stir in and deglaze everything with the vegetable stock. Reduce by a third over medium heat and then pour in 200 ml of cream. Simmer briefly and season the sauce with salt, pepper and lemon juice. Add the chicken and let it steep for another 5 minutes over a mild heat.
Boil pasta, rinse cress.
Serve the pasta with the sauce and sprinkle the fresh cress on top.