Mix flour, eggs, roughly enough carbonated mineral water to form a smooth dough, salt, pepper, and mace together. Using a spaetzle press, pipe the dough into boiling salt water. Cook for 3-5 minutes, stirring occasionally with a wooden spoon to prevent sticking, until the spaetzle floats to the surface. Remove with a slotted spoon and drain well.
Chop or grate the mountain cheese, farmer's cheese, Romadur, and Limburger; mix together. Layer the mixture over the drained spaetzle. Bake in the oven at 180°C (356°F) for 10-15 minutes until the cheese melts and reaches your desired browning level.
While the cheese bakes, cut the onions into fine strips. Heat butter in a pan over medium heat and fry the onions for 10-12 minutes, stirring occasionally, until they turn golden brown and crispy. Spread the caramelized onions over the finished cheese spaetzle.