In large flameproof casserole, dry-fry lamb over medium-high heat until lightly browned, 6 to 8 minutes, stirring frequently. Add carrots, celery, and leeks. Cook 4 minutes, stirring occasionally.
Add cider, broth, and salt and pepper to taste. Bring to a boil. Reduce heat to low, cover and simmer until vegetables are tender, 20 to 25 minutes.
Meanwhile, heat oven to 400ºF. Prepare pastry topping: In medium bowl, stir together flour, parsley, sage, salt, and pepper. Stir in sour cream and mix until firm dough forms. If dough is too dry, add 1 or 2 teaspoons milk. Roll out dough to 1/2-inch thickness. Cut into 16 triangles.
Add peas and bouquet garni to casserole. Arrange triangles on top, covering surface.
Bake until topping has risen and golden brown, 25 to 30 minutes.