Heat the margarine, dust it with flour and stir until a yellow mass is formed. Gradually pour in the stock and milk, while always stirring vigorously with the whisk so that no lumps form. Let the sauce cook, stirring occasionally, until it becomes creamy, then season.
Lightly fry the finely diced onions in a little oil, add the very finely chopped mushrooms and cook for a few minutes over medium heat with the lid closed. Finely grate the carrots and add to the pan. Season the vegetables and add to the sauce, seasoning if necessary.
The sauce tastes good with various side dishes and can be prepared with different vegetables, such as zucchini, corn, kohlrabi, etc.