In a large pan, heat up the oil, add half of the butter and allow it to melt.
Season the chicken breasts with salt and pepper, and sear them on both sides until golden.
Remove them from the pan and set aside.
In the same pan, melt the remaining butter, add the peeled and chopped onions and fry until golden.
Return the chicken to the pan.
Add the flour to a ramekin together with 2 tablespoons of water, and mix well to have a lump-free mixture, then pour it over the chicken together with the chicken stock/broth.
Cook the chicken with the lid on for 20 minutes until it's cooked through.
Add the creme fraiche, lemon juice and zest, and mix well to get a smooth sauce.
Check if it needs any more seasoning, then garnish with parsley.
Serve with mashed potatoes and asparagus.