Cut the onion and fry it in a little oil in a large saucepan. Add the minced meat and fry until crumbly. Add the mustard and tomato paste, you have to experiment a little with the amount. I always take as much as in the recipe, but that also depends on the mustard, if it tastes very intense, it is better to take less!
Pour in the broth until a creamy sauce is formed. Clean the leek and cut into rings. Add to the minced meat and cook until soft. If necessary, season with salt and pepper.