Wash and clean the fennel and cut into thin slices, keeping the fennel greens as a garnish.
Heat butter in a pan or saucepan and sweat the diced onions until golden. Then add the fennel, fry a little and sauté with a little liquid (wine, if necessary a little water). Season with a little salt and pepper and possibly some grainy broth.
Those who prefer to add cream instead of wine should do so at the end of the cooking time.
After approx. 15 - 20 minutes, when the fennel is soft but still firm to the bite, stir 1 - 3 tablespoons of sweet mustard into the vegetables. That gives a sweetish, spicy taste.
Sprinkle the chopped fennel greens on top and serve.
I have either fried meat, fish or just bread with it. As a side dish, the recipe is designed for approx. 4 people, as a main course with bread for 2 people.