Heat 2 tablespoons oil in a large Dutch oven over medium-high heat. Season the lamb with 1/2 teaspoon salt and 1/2 teaspoon pepper. Working in batches, add to the pot and cook, stirring occasionally, until lightly browned, about 4 minutes. (The lamb should be rare at this point.) Using a slotted spoon, transfer the lamb to a plate.
Add the stock to the pot and bring to a boil, stirring to release the browned bits in the bottom of the pot. Pour the stock from the pot into a large glass measuring cup or heatproof bowl. Transfer 1/2 cup stock to a small bowl and stir in the harissa to make a very spicy seasoning sauce for the finished tagine. Set the harissa sauce aside. Reserve the remaining stock.
Add the remaining 2 tablespoons oil to the pot and heat over medium heat. Add the onion and carrots and cook, stirring occasionally, until the onion is golden, about 6 minutes. Stir in the garlic and cook until fragrant, about 1 minute. Stir in the ginger, cinnamon, and hot pepper flakes and cook for 15 seconds. Stir in the tomatoes, then the reserved chicken stock, and bring to a boil. Reduce the heat to low, cover, and simmer until the carrots are tender, about 20 minutes. Return the lamb and any collected juices to the pot, and stir in the garbanzo beans. Increase the heat to medium-high and cook just until the beans are heated through, about 5 minutes. Season with salt and pepper to taste.
To serve the lamb tagine, carefully transfer the roasted pumpkin to a platter. Remove the lid, ladle in the tagine, and replace the lid. (Do not worry if all of the tagine won't fit into the pumpkin; simply reserve it in a serving bowl.) Present the pumpkin at the table. Spoon the lamb tagine onto individual rimmed plates or shallow bowls. When the tagine is served and the pumpkin is empty, cut the pumpkin into wedges and place them alongside the tagine. Serve the lamb tagine immediately, with the couscous and harissa sauce passed on the side.