Cut onion into chunks. Heat some oil in a large saucepan over medium-high heat. Fry onion for 3-4 minutes until translucent. Add minced meat and fry for 5-8 minutes, breaking it apart as it cooks, until crumbly and browned.
Stir in ½ tube mustard and ½ tube tomato paste. Pour in broth until a creamy sauce forms, then bring to a simmer.
Clean leeks and cut into rings. Add to the pan and simmer on medium heat for 10-15 minutes until leeks are soft. Season with salt and pepper to taste.