Bring vegetable broth to a simmer over medium heat in a pot. Add 3 Maultaschen (60 g each) and maintain a gentle simmer for approximately 8-10 minutes.
While the Maultaschen cook, cut 3 red onions into small pieces. Heat a knob of butter in a separate pan over medium-high heat. Add the onions and sauté for 4-5 minutes until they become translucent. Season with sea salt and freshly ground pepper.
Add 0.5 teaspoon sugar to the onions. Cook for 2-3 minutes more, stirring occasionally, until the onions caramelize.
Remove the caramelized onions from the pan and set aside.
Remove the Maultaschen from the broth with a slotted spoon. Toss them in the pan with a knob of butter and season with freshly ground pepper.
Plate the Maultaschen, top with the caramelized onions, and serve with lamb's lettuce.