Boil the sweet potatoes until tender (about 15 minutes). Drain and mash with coconut oil and plain Greek yogurt (if using)., In a large skillet, brown the ground lamb until fully cooked. Drain off excess fat., Add peas, carrots, and spinach to the skillet, cooking until vegetables are tender., Preheat your oven to 350°F (175°C)., In a baking dish, spread the lamb and vegetable mixture evenly across the bottom., Spoon the mashed sweet potatoes on top, spreading evenly to cover the meat and vegetables., Bake in the oven for 20 minutes or until the top is slightly golden brown., Let the pie cool completely before serving. Store leftovers in the fridge for up to 3 days.