Preheat and Melt Fat: Preheat your casserole or pot over medium heat. Melt a little lamb fat in it.
Sear the Lamb: Sear pieces of lamb shoulder or leg for 2 to 3 minutes on both sides until they brown nicely. Then drain the fat.
Sweat the Vegetables: Sweat the onions, carrots, and celery in the remaining fat, seasoning with salt, and cook for 4 to 5 minutes.
Add Garlic and Tomato Concentrate: Add crushed garlic cloves and a tablespoon of tomato concentrate. Stir over low heat for 2 to 3 minutes.
Deglaze and Add Broth: Deglaze with white wine or lemon juice. Moisten with 50 cl of broth, ensuring to scrape off the cooking sugars from the pot.
Add Lamb and Herbs: Return the lamb pieces to the pot, add thyme and bay leaf, cover, and let simmer on low heat for 4 hours.
Add Potatoes and Optional Vegetables: After 15 minutes, add potatoes (and mushrooms if using) cut into 5 cm pieces. Cover and let simmer for an additional half hour.
Finish and Serve: If needed, adjust the cooking time until the lamb is tender. Garnish with tarragon or parsley before serving.