Bring 2 inches of water to a boil in a large pot filled with a steamer basket. Add potatoes and steam until tender, 10 to 12 minutes. Drain and return the potatoes to the pot. Add milk, butter and 1/4 tsp. each of salt and pepper. Mash together to a chunky consistency. Cover and keep warm. Meanwhile, heat oil in a large nonstick skillet over medium-high heat. Add lamb, onion, carrots and the remaining 1/4 tsp. each of salt and pepper; cook, stirring until the lamb is no longer pink, 6 to 8 minutes. Sprinkle flour and oregano over the mix and cook, stirring, until thickened, 3 to 5 minutes more. Ladle the lamb into 4 bowls and top with potatoes. Serves 4.