Brown ground lamb in large skillet over medium-high heat, stirring occasionally. Stir in onion, celery, carrot and garlic and cook, stirring occasionally, until vegetables are tender, about 10 minutes.
Cover potatoes with water in 2-quart saucepot; bring to a boil over medium-high heat. Reduce heat and simmer 15 minutes or until potatoes are tender; drain. Return potatoes to saucepot, then add Hellmann's® or Best Foods® Real Mayonnaise and milk. Mash potatoes until smooth and creamy. Season, if desired, with ground black pepper and set aside.
Stir flour into lamb mixture and cook over medium heat, stirring frequently, 2 minutes. Stir in tomato paste, water and Knorr® Homestyle Stock – Beef and bring to a boil over high heat. Reduce heat and simmer gently, stirring occasionally, 20 minutes or until mixture thickens.
Spoon lamb mixture into 2-quart shallow oven-proof casserole, then top with mashed potatoes. Broil until lightly golden, about 1 minute. Sprinkle with parsley.