Wash the chicken fillets and cut into roughly 3 cm pieces. Place in a container and season with salt and pepper to taste.
Cut the small onion into half rings and add to the chicken.
Pour 1.5 to 2 cups of kefir over the chicken to cover, adjust salt to taste, cover, and refrigerate for at least 2 hours or overnight.
Heat a frying pan over medium-high heat and add vegetable oil.
Fry the marinated chicken in portions on all sides until golden brown, about 5-7 minutes per batch, until the internal temperature reaches 74 °C (165 °F).