Heat 2 tablespoons olive oil in a large skillet over medium-high. Sprinkle lamb with 2 1/2 teaspoons kosher salt and 1 teaspoon black pepper. Cook until browned, about 3 minutes per side. Remove lamb from skillet. Add onions, carrots, and garlic; cook, stirring occasionally, until slightly softened, about 4 minutes. Add 1/2 cup dry red wine; cook, until mostly evaporated, about 3 minutes. Transfer mixture to a 6-quart slow-cooker. Stir in tomatoes, chicken broth, and ground cumin. Add lamb shanks, nestling into tomato mixture. Cover and cook on LOW until lamb is very tender, about 6 hours.

Transfer lamb shanks to a plate. Pour cooking mixture through a wire-mesh strainer into a bowl; discard solids. Reserve strained cooking liquid for another use, if desired. Shred lamb and set aside, covered.

Heat 2 tablespoons olive oil in a 10-inch cast-iron skillet over medium-high. Add chopped yellow onion and carrots; cook until tender, about 10 minutes. Add tomato paste, rosemary, chopped garlic, thyme, 1 1/2 teaspoon kosher salt, and 3/4 teaspoon black pepper. Cook until fragrant, about 2 minutes. Add 1/2 cup dry red wine; cook until completely evaporated, about 2 minutes. Sprinkle with 3 tablespoons all-purpose flour; cook 1 minute. Gradually add beef stock, stirring until completely incorporated. Cook until thickened, 6 to 7 minutes. Stir in 12 oz. shredded reserved lamb shank meat and green peas. Remove from heat.

Stir heavy cream into prepared mashed potatoes. Spread potatoes over lamb mixture, covering surface completely. Broil on HIGH 6 inches from heat until lightly browned, 4 to 6 minutes. Garnish with chopped parsley.