Preheat oven to 350°F (175°C). Grease a 9x13-inch (or similar) baking dish.

Bring a large pot of salted water to a boil. Cook spätzle according to package instructions: fresh spätzle 2–4 minutes, dried 8–10 minutes. Drain and toss with 2 tbsp butter.

Melt 2 tbsp butter in a large skillet over medium heat. Add sliced onions and a pinch of salt; cook, stirring occasionally, until deeply golden and caramelized, 15–20 minutes. Reduce heat if they brown too fast.

In a saucepan, combine cream and milk over low heat. Stir in Dijon, nutmeg, salt, and pepper. Warm until just simmering, about 3–4 minutes. Do not boil.

Remove saucepan from heat and add grated Emmental and Gruyère a handful at a time, stirring until smooth and melted — about 2–3 minutes. If using eggs for extra silkiness, temper them: whisk a few tablespoons of hot cream into beaten eggs, then stir the egg mixture into the sauce off heat.

Toss the cooked spätzle with the cheese sauce until evenly coated. Taste and adjust salt/pepper.

Layer half the spätzle-cheese mix into the prepared baking dish. Sprinkle half the caramelized onions and a little more cheese. Add the remaining spätzle and top with remaining onions and extra cheese.

Bake at 350°F (175°C) for 15–20 minutes, until cheese is melted and edges are bubbling. For a golden top, broil 1–2 minutes — watch closely.

Remove from oven, let rest 5 minutes, then sprinkle with crispy fried onions and chopped chives. Serve hot.

Skip the oven and serve immediately after tossing spätzle with the cheese sauce, topping with caramelized and crispy onions.