Moroccan Lamb Tagine

Name: Moroccan Lamb Tagine
Ingredients: Good olive oil; 6 small frenched lamb shanks (5 to 6 pounds total); 3 cups chopped yellow onions (2 large onions); 3 garlic cloves, thinly sliced; 1 tablespoon grated fresh ginger; 1 1/2 teaspoons chili powder; 1 1/2 teaspoons ground turmeric; 1 1/2 teaspoons ground cumin; 1/2 teaspoon ground cardamom; 1 (4-inch) cinnamon stick; 1 (28-ounce) can diced tomatoes, such as San Marzano; 2 cups good chicken stock, preferably homemade; 2 tablespoons light brown sugar, lightly packed; 4 (1/2-inch-thick) slices of lime; Kosher salt and freshly ground black pepper; 1 pound Yukon Gold potatoes, unpeeled and 1-inch-diced; 1 pound butternut squash, peeled and 1-inch-diced; 1/2 pound sweet potatoes, unpeeled and 1-inch-diced; Couscous, for serving