Cut the sausages, mushrooms and the feta cheese in small pieces.
Fry the sausage with the olive oil in a frying pan.
Boil the spätzele according to the instruction on the bag.
Add the mushrooms and the salt in the frying pan and stir.
Lower the heat on the frying pan and add the tomato sauce.
Add ½ of the feta cheese in the frying pan and continue to cook another 5 minutes.
Serve the spätzele and the sauce hot on a plate.
Sprinkle the rest of the feta cheese and the arugula on the dish.