Mix flour, eggs, milk and salt with a wooden spoon or a food processor for 10 minutes until you get a homogeneous paste. Leave to rest 1 hour. Tips : After 1 hour you can see bubble if you mix really quickly with a wooden spoon from top to bottom.
Peel and slice onions. Heat butter on a large pan, add onions and brown. Reserve.
Grate all cheese and reserve.
Preheat oven to 120°C.
Bring to boil a large casserole dish. Place the Spätze-grater on top of the casserole. Pour some of the spaetzle paste and move back and forth quickly or more slowly depending on the size of the spaetzle you want to make.
When they come to the surface, they are done !
Put them little by little in a oven dish covering them with cheese and onions from time to time until you have no more ingredients left.
Mix, cover and put in your oven from 5 to 10 minutes to get enough time to wash your Spätzle-grater and serve !