The key to authentic Käsespätzle is the dough: it should be elastic and contain enough eggs to make the Spaetzle fluffy. The cheese combination is also important – Bergkäse (mountain cheese) provides a sharp flavour, while Emmentaler adds creaminess and stretchiness.
When mixing hot Spaetzle with cheese, traditional cooks add a little hot water from the Spaetzle cooking water or vegetable broth. This helps the cheese melt into a creamy sauce (known as Schmelzkäse).
In a bowl, mix flour, eggs, salt, and nutmeg. Gradually add water/milk until you get a thick yet elastic dough – it should be able to drip but not be runny.
Knead the dough thoroughly (whisk/beat) with a wooden spoon or mixer for 5–10 minutes until smooth and bubbles begin to form.
Cover the dough and let it rest for at least 30 minutes.
Slice the onions into thin rings or half-rings.
Melt the butter in a pan and slowly fry the onions over medium heat until they are golden brown and crispy. This takes 10–15 minutes. Remove the finished onions and set aside, but keep the fat in the pan.
Bring a large amount of salted water to a boil in a large pot.
Press the dough into the boiling water using a special Spaetzle board (Spätzlebrett), press (Spätzlepresse), or sieve (Spätzlehobel).
As soon as the Spaetzle float to the surface (approx. 1–2 minutes), remove them with a slotted spoon and let them drain. Important: Save a little hot cooking water (about 100 ml).
In the pan where you fried the onions (or in a preheated baking dish), place a layer of hot Spaetzle and sprinkle with a portion of the grated cheese.
Alternate layers of Spaetzle and cheese. After adding the final layer of cheese, pour in the reserved hot water/broth (or a little milk).
Stir over low heat (or bake briefly in the oven) until the cheese is completely melted and forms a creamy, stringy sauce that coats the Spaetzle.
Serve immediately, generously topped with the fried onions (Röstzwiebeln) and chopped chives.