Cut the chicken fillet in half.
Place the chicken fillet in a deep container. Add the Provencal herbs, chicken spices, salt and pepper to taste.
Pour kefir over the chicken fillet with seasonings and stir.
Peel and chop the onion and garlic and add to the chicken in kefir. Mix and leave for 10 minutes so that the chicken fillet is soaked in the marinade.
Place the marinated chicken fillet in a baking dish.
Bake the chicken in kefir for 60 minutes in an oven preheated to 180 degrees.