Grilled goat cheese burger with onion jam and toasted walnuts

For the onion jam, peel and thinly slice or grate the onions.

Sauté in a little oil, season with salt and sugar, add the vinegar and star anise, then cook until it reaches a jam-like consistency.

For the vinaigrette, mix the mustard and 0.5 dl of olive oil with an immersion blender; you can season it with a pinch of salt and sugar, or a spoon of honey.