Cook the ribbon noodles in salted water according to the package directions.
Pat dry the canned mushrooms and cut into slices.
Peel and dice the onion.
Mix the flour with salt and pepper in a shallow plate or bowl.
Wash the chicken fillets and pat dry, then coat each one in the seasoned flour mixture.
Heat 2 tablespoon of butter in a large pan over medium heat. Fry the chicken fillets for 8 minutes on each side until golden and cooked through (74°C / 165°F internal temperature).
Add the diced onion and sliced mushrooms to the pan. Cook for 5 minutes, then cover and set aside.
In a saucepan over medium heat, melt the remaining 2 tablespoon of butter. Cook the remaining flour in the butter for 1–2 minutes, stirring constantly.
Gradually pour in the chicken broth while stirring constantly. Simmer for 5 minutes.
Remove from heat. Stir in the sour cream and season with mustard, salt, and pepper.
Pour the sauce over the chicken, onion, and mushrooms and mix gently to combine.
Serve the chicken and sauce with the ribbon noodles. Optional: add a splash of dry white wine to the sauce for extra flavor.