Combine flour, eggs, milk, and sparkling mineral water in a mixer. Mix until smooth. Season to taste with salt.
Press the batter through a dumpling sieve into a pot of boiling salted water. Cook for 1-2 minutes until spaetzle floats to the surface. Drain in a fine-mesh sieve and rinse with cold water to stop cooking.
Cook the bacon and leeks in oil over medium-high heat until bacon is crispy, about 5-7 minutes. Add the cooked spaetzle and stir well. Pour in sour cream and mix to combine. Add grated cheese and season with salt, pepper, and nutmeg to taste.