Heat the butter in a pan. Add the onion and garlic and let it turn to glass.
Add the cleaned and sliced mushrooms. Simmer over medium heat until the mushrooms take on color. Pour in the cream and simmer on a low heat.
In the meantime, cook the spaetzle according to the instructions on the packet. If you like, you can also fry them briefly in butter afterwards. Season the finished mushrooms with salt, pepper and parsley and arrange them together.