Heat oven to 375°F.
Place 2-quart metal oval au gratin pan or 10-inch oven-proof skillet over medium-high heat. Add lamb; cook, stirring to break up clumps, until browned, about 3 minutes. Add onion, celery and carrot; cook 8 minutes. Remove pan from heat.
In large bowl, combine sour cream, eggs, dill, flour, salt and pepper. Stir peas and sour cream mixture into lamb until thoroughly mixed.
In same large bowl, combine thawed mashed potatoes, sour cream, egg, dill, salt and pepper. Shape potato mixture into ovals with spoons. Arrange or spread on top of lamb mixture.
Bake in 375°F oven until top is slightly browned and filling is bubbly, about 40 minutes. Let stand 10 minutes before serving.
