Start by making your spaetzle batter so that it’s ready to go when you need it. You can find the recipe for that in the link in the description above.
Heat olive oil in a large skillet over medium high heat. Add onions and cook until just beginning to soften, then add in mushrooms and stir to combine. Continue to cook until mushrooms begin to brown and release their juices. Add in minced garlic for the last minute. Once everything is cooked, set skillet aside for later.
In a large saucepan, heat butter until just beginning to foam. Dump in flour and whisk constantly. This mixture will turn into a paste that you’ll keep whisking for a minute or two longer until it turns a little brown. Congratulations! You’ve just made a roux!
Now add the milk by the 1/2 cup, whisking constantly with each addition. Let the mixture thicken for a minute or two on high heat, continuing to whisk so that it doesn’t stick to the bottom of your pot.
Dump in the grated swiss, stirring constantly until completely melted and incorporated into a smooth sauce. Set the sauce aside.
Boil your spaetzle, according to instructions. Combine fresh spaetzle, mushroom mixture, and Swiss sauce and enjoy!