Heat 1 tablespoon olive oil in a large skillet. Add the onions, cook for a few minutes, then add the carrots and leek. Cook for about 5 more minutes until the veggies begin to soften. Transfer these veggies into the slow cooker, and add the potatoes and cabbage in a layer on top of the sauteed vegetables.
Return the skillet to the stove, heat the remaining olive on medium. Add the lamb. Salt and pepper the lamb to taste, then cook for 5 minutes on one side to brown. Turn the lamb over, brown the other side for another 5 minutes.
Transfer the lamb to the slow cooker. Pour in the chicken stock and the water, then add the sprigs of rosemary and bay leaf.
Cook in the slow cooker for 1 hour on high, 8 hours on low. When cooked, stir in the baby spinach leaves for some nice green color.
Serve hot, topped with a touch of fresh mint, along with a pint or glass or hearty red wine. Sláinte!