1. For the Spätzle, mix flour and salt in a bowl. Beat the eggs and mix with the flour, using a kneading attachment of an electric mixer. Gradually add water and soda water.
You might have to add a few tablespoons more flour. Knead the dough until it produces bubbles and easily detaches from the sides of the bowl. Cover the bowl with a dishtowel and let the dough rest for 30 minutes.
2. Preheat the oven to 400 degrees F.
3. For the topping, melt the butter in a large skillet and sauté the onions until brown and crisp.
4. Bring salted water to a boil in a large pot. Reduce the heat. Spread about 1/6 of the dough on the short end of a large cutting board and scrape it off the board into the boiling water, using a knife. Make sure that you cook only small batches at a time. Simmer until the Spätzle float on the surface.
5. Remove the Spätzle and place them into a colander. Rinse them with cold water and place them in the prepared dish. Sprinkle with pepper and a cheese, cover with aluminum foil and place in the oven.
6. Increase the heat to 450 degrees F.
7. Top the last layer of Spätzle with sautéed onions and the remaining cheese and bake uncovered for 5 to 7 minutes.
Source: Nadia Hassani: “Spoonfuls of Germany” (Hippocrene Books, 2004)