Sauté Vegetables: Heat the olive oil in a non-stick skillet over medium heat. Add the diced onion and minced garlic, sautéing until soft and translucent. Add the sliced leek and celery, cooking until softened.
Slow Cooker: Transfer the sautéed vegetables to the slow cooker. Add the red lentils, bay leaves, and chopped carrots.
Add Gammon and Stock: Place the smoked gammon joint on top of the lentils and vegetables. Pour the stock over the ingredients, ensuring the gammon is just covered.
Cook: Cover and cook on high for 3-4 hours or on low for 5-6 hours, until the ham is thoroughly cooked.
Prepare Ham: Remove the gammon joint from the soup and let it rest on a cutting board for a few minutes. Shred the ham using two forks, leaving some larger chunks.
Blend Soup: Use a stick blender to gently blend a portion of the soup to thicken it, maintaining some texture.
Add Greens: Stir in the cavolo nero or kale and allow it to wilt in the hot soup for about 5-8 minutes.
Combine and Serve: Return the shredded ham to the soup, stir well, and adjust the seasoning as needed. Serve hot with bread and butter.