Put 1 tablespoon of butter and oil in a frying pan, on high heat until hot.
Saute onions and mushrooms together for several minutes until soft and caramelised.
Add 3 tablespoons of flour and stir until mixed throughout. Cook for about 1 minute.
Add the dry white wine and stir. It will start to bubble and evaporate.
Slowly add the chicken stock in small amounts and stir over medium heat. Sauce will start to thicken.
Add thickened cream and simmer on low for approx 5 minutes, and take off heat.
Place kotleti in a baking dish. Pour mushroom sauce over the kotleti and cover with foil and bake on 350°F for about 10-15 minutes until heated through. Alternatively, serve kotleti with mushroom sauce on the side