To prepare the lamb shanks, preheat the oven to 250°F.

Heat a Dutch oven over medium-high heat and pour in the olive oil.

Season the lamb shanks with salt and pepper. When the oil is hot, add the shanks and brown on all sides. Add the carrots, celery, onion, garlic, stock, wine, pomegranate juice, bay leaf and peppercorns and bring to a simmer

Cover the pot tightly, transfer to the oven and cook until the meat is fall-off-the-bone tender, 2 1/2 to 3 hours, flipping the shanks once during cooking.

Remove the pot from the oven and let the lamb rest at room temperature until cool enough to handle, about 45 minutes, then remove the meat from the bones, discarding the bones. Strain the braising liquid. Cover and refrigerate the meat and braising liquid separately overnight.  

To prepare the ragout, in a Dutch oven over medium-high heat, warm the olive oil. Add the onions, carrots and celery, reduce the heat to medium and cook, stirring occasionally, until soft. Add the mushrooms and cook, stirring occasionally, until soft and beginning to release their liquid. Add the tomato paste and cook until it turns brick-red in color, about 1 minute. 

Skim the fat off the top of the reserved braising liquid and add the liquid to the pot. Simmer until reduced by half, then add the reserved lamb. Simmer slowly until the meat is hot and begins to shred. Season to taste with salt and pepper. Remove from the heat and keep warm.

While the ragout simmers, prepare the pumpkin polenta: In a large saucepan over high heat, combine the stock and milk and bring to a boil. Whisk in the polenta, reduce the heat and cook, stirring frequently, until the polenta is thickened and not grainy in texture, about 30 minutes. Stir in the pumpkin puree.

Remove from the heat and whisk in the mascarpone and butter. Season to taste with salt and keep warm.

To prepare the kale salsa verde, in a blender, combine the kale, parsley, garlic, capers, lemon zest, lemon juice, vinegar and olive oil and blend on high speed until smooth. Season to taste with salt.

To prepare the spiced pumpkin seeds, preheat the oven to 300°F.

In a small bowl, toss the pumpkin seeds with the salt and chili powder. Transfer to a baking sheet and spread in a single layer. Toast in the oven until fragrant, 3 to 5 minutes. Let cool to room temperature, then roughly chop.

To serve, spoon the polenta into individual bowls. Top with the lamb and mushroom ragout. Spoon the salsa verde over and around the ragout. Sprinkle the spiced pumpkin seeds on top.