In the meantime, peel and dice the onion, clean the mushrooms and cut into slices. Let the oil get hot, add the onions and sauté until translucent. Add the mushrooms and cook for a short time. Add the butter vegetables and toss them through briefly. Now deglaze with milk and / or cream and a little vegetable stock. Simmer until the vegetables are cooked through. Season to taste with salt and pepper and, if necessary, some vegetable stock powder. Stir in the sauce thickener and bring to the boil again briefly.