Remove the casings from the sausages and cut them into strips. Heat oil in a large skillet over medium-high heat. Fry the sausage strips until browned, about 3–4 minutes.
Add the diced onions to the pan and fry for 2 minutes. Quarter the mushrooms, add them to the pan, and sauté everything together for about 5 minutes until the mushrooms are tender.
In a small bowl, whisk together the mustard, cream, cornstarch, and marjoram. Pour the mixture into the pan and stir well. Bring to a boil. Season with salt and pepper to taste.