Mix the soy schnitzel with the soy sauce and then soak in the vegetable stock for about 10 minutes. Lightly squeeze out excess moisture before further processing.
Cut the onions and mushrooms into thick slices. Heat the butter and olive oil in a large pan. Add the soy schnitzel and onions. Fry for about 5 minutes until the schnitzel is lightly browned and the onions are soft. Stir in mushrooms and fry for approx. 2 minutes. Pour in the milk and reduce, stirring, until almost evaporated. Stir in the cream (or sour cream) and spices, heat and season to taste.
At the very end, add lemon juice and serve immediately with pasta, rice or potatoes.