Put the butter in a large skillet over medium heat. When it foams, add the onions and mushrooms and sprinkle with salt and pepper. Cook, stirring occasionally, until they are soft but not browned, 10 to 15 minutes.
Add the beef and cook, stirring, for just a minute. Add the mustard and stock and adjust the heat so the liquid bubbles steadily. Cook, stirring occasionally until the meat is just cooked through but still tender, about 5 minutes.
Add the sour cream and stir until it heats through and forms a sauce, careful not to let it come to a boil. Taste, adjust the seasoning, garnish with dill, and serve.