Cook the Spätzle in plenty of boiling salted water for 15-18 minutes. In the meantime, clean and wash the leek and cut it into approx. 5 mm thick rings. Blanch the leek in boiling salted water for about 1 minute.
Remove and put aside. Wash, clean and quarter the tomatoes and remove the seeds. Then cut into medium-sized cubes. Peel onion and cut thin rings. Heat 1 tablespoon of oil in a pan and fry the onion rings in it for about 10 minutes at medium heat until golden brown.
Grate the cheese roughly. Drain the spaetzle and let it drain. Heat the remaining oil in a pan and fry the spaetzle for 2-3 minutes. Spread cheese on the spaetzle, let it melt for a short time and deglaze with broth.
Season with salt and nutmeg. Add leek and stir in. Add tomato cubes and arrange in a casserole dish. Sprinkle with the roasted onions. Serve with lettuce with herb vinaigrette.