Cut the onion into half rings and fry with mushrooms until tender. Salt and season to taste. At the very end, add finely chopped garlic and stir.

Prepare sauce according to package instructions. Milk or additional water may be needed. Or use ready-made cream from a jar.
Grate the cheese and add it to the sauce when it is ready. Mix well.

Place mushrooms and onions on potatoes in an even layer. Top with cream cheese sauce. Bake in an oven preheated to 180 degrees for 10-15 minutes until golden brown.

Let chill on the table for a while before serving. The casserole will stabilize, cool a little and can be sliced ​​beautifully.