Heat olive oil in a skillet over medium heat.
Season the chicken with salt and pepper, then add it to the skillet.
Cook chicken for about 5-7 minutes or until browned, then remove from the skillet.
In the same skillet, add the diced onions and mushrooms; sauté until softened.
Sprinkle flour over the onion and mushroom mixture and stir to combine.
Gradually pour in the chicken broth, stirring constantly to avoid lumps.
Add the Dijon mustard and stir until fully combined.
Return the chicken to the skillet and simmer for about 10 minutes.
Stir in the sour cream and heat gently until warmed through; do not boil.
Adjust seasoning with salt and pepper, then serve over noodles or rice with parsley on top.
