Marinate the thighs. Prepare the marinade. Mix garlic, salt, lemon juice, mustard, and hot pepper. Rub the chicken fillet with the marinade and refrigerate for 30 minutes.
While the chicken is marinating, prepare the filling. Fry the onion in sunflower oil in a frying pan until golden brown for 5 minutes, add the champignons, fry for another 10 minutes. Add cheese and herbs to the mushrooms. Salt, pepper, mix. Fry for 5-7 minutes.
Stuff the chicken. Place the chicken thigh skin side down, add the mushroom stuffing. Secure the edges of the stuffed thigh with toothpicks. Place the thighs with mushrooms in a baking tray. Bake the stuffed chicken thighs in the oven for about 40-45 minutes at 180-190 degrees.