Heat the olive oil in a large skillet over medium-high heat. When you begin to see wisps of smoke, add the lamb to the pan in a single layer. Sprinkle with 1 teaspoon of the salt. Cook until the lamb is deeply browned on all sides, 15 to 20 minutes. Transfer the meat to a 5- to 6 ½-quart slow cooker using a slotted spoon, reserving the fat in the pan.
Reduce the heat to medium and add the chard stems (reserving the leaves to add later), onions, and garlic to the pan (add a little more olive oil if necessary). Cook, stirring occasionally, until the vegetables are just tender and lightly charred, 3 to 5 minutes. Stir in the cardamom and pepper and cook until distributed evenly and aromatic, about 1 minute. Transfer the vegetable mixture to the slow cooker. Add the water to the sauté pan and scrape the bottom to release the flavorful stuck-on bits, then pour it into the slow cooker.
Add the butternut squash to the slow cooker, and pour in the apple cider. Submerge the cinnamon stick, bay leaf, and cloves in the liquid, and season with the remaining 1 teaspoon of salt. Set the slow cooker to the low setting and cook until the meat is tender and shreds easily with a fork and the vegetables are very tender, 6 to 8 hours.
Discard the bay leaf and cinnamon stick. Gently stir in the chard leaves and continue cooking until wilted and just tender, 5 to 10 minutes. Serve the stew in warm shallow bowls topped with a dollop of yogurt and sprinkled with cilantro.