In a large frying pan over medium-high heat, warm oil. Add onions and garlic, and sauté until golden, about 5 minutes. Stir in ginger, cumin, cayenne, and turmeric and sauté until fragrant, about 30 seconds. Pour in broth, raise heat to high, and deglaze the pan, stirring to scrape up the browned bits on the bottom. When broth comes to a boil, remove pan from heat.
Put lamb in a slow cooker, and sprinkle with 1 tablespoon salt. Add contents of frying pan. Cover and cook on high-heat setting for 4 hours or low-heat setting for 8 hours.
Add baby spinach to pot and cook, stirring occasionally, until spinach is wilted, about 5 minutes. Just before serving, stir in 1 1/3 cups yogurt. Season to taste with salt. Spoon into shallow bowls and serve, passing remaining yogurt to add as a garnish.
Try another Crock-Pot recipe: Tarragon Chicken
Adapted from Williams-Sonoma Food Made Fast: Slow Cooker, copyright © 2006 by Weldon Owen, Inc., and Williams-Sonoma, Inc., with permission from the publisher, Oxmoor House.