Beat eggs and about 1 cup milk in an electric mixer fitted with the paddle attachment. Add 2 cups flour and mix for a minute, then add the remaining flour. The batter will be very thick. Add about 1 cup more milk and beat until glassy and thick. It's the right consistency when it takes 4-5 seconds to drip off a wooden spoon. If it's too thin, add some additional flour. If it's too thick to drip, add more milk.
While you're mixing up the dough (or after, it's okay if the dough sits), chop your onions to small dice. Melt butter in a frying pan and cook the onions until translucent, but don't caramelize them. Set aside. (If you don't want to serve with onions, skip this step, but you'll still need the butter.)
Meanwhile, begin boiling your water. It is best to use a large stock pot that has a nested colander. Once water is boiling, add approximately a cup of batter to the spaetzle press and slowly slide the press back and forth to create your knoepfle. Watch for the knoepfle to float to the top of the water, stir to get any that are stuck on the bottom to float, then boil for an additional one minute (after they are all floating). Strain and place into a large bowl. Bring water back up to a boil between batches.
Once adding a batch to your serving bowl, add either some of the onions or 2 tablespoons of butter. Stir so they don't stick together.
Serve with the sauted onions, extra butter, gravy, and Swiss or parmesan cheese.
These freeze well if you have leftovers.
Video
Notes
The ratio for ingredients is approximately 1 egg : 1 cup flour. It's easy to up how much you make by simply using more eggs and flour following that ratio (and more milk).