Pat the lamb cubes dry with paper towels. Season generously with 1 teaspoon of salt and 0.5 teaspoon of black pepper. Heat 2 tablespoons of olive oil in a large skillet or Dutch oven over medium-high heat. Brown the lamb in batches until nicely seared on all sides, about 3-4 minutes per batch. Do not overcrowd the pan. Remove the browned lamb and set aside.

Reduce the heat to medium. Add the finely chopped onions to the same skillet, adding a splash more oil if needed. Cook, stirring occasionally, until softened and translucent, about 5-7 minutes. Add the minced garlic, grated ginger, and minced green chili (if using) and cook for another 1-2 minutes until fragrant.

Add the garam masala, ground coriander, ground cumin, turmeric powder, and Kashmiri chili powder (or paprika) to the skillet. Stir constantly for 1 minute until the spices are very fragrant. Add the tomato paste and cook for another minute, stirring to combine with the spices and aromatics.

Transfer the browned lamb to the slow cooker insert. Pour the onion-spice mixture from the skillet over the lamb. Add the undrained diced tomatoes, cinnamon stick, crushed cardamom pods, and bay leaf. Stir gently to combine.

Pour in the chicken or beef broth. Cover the slow cooker and cook on LOW for 7-8 hours or on HIGH for 3.5-4 hours, until the lamb is fork-tender.

Once the cooking time is complete, remove the cinnamon stick, cardamom pods, and bay leaf. Stir in the full-fat Greek yogurt. If the sauce is too thin, you can remove the lid and cook on high for an extra 30 minutes, or thicken with a cornstarch slurry (1 tbsp cornstarch mixed with 2 tbsp water) added during the last 30 minutes of cooking.

Ladle the slow cooker lamb curry into bowls. Garnish generously with fresh chopped cilantro. Serve hot with steamed basmati rice or warm naan bread.