Heat the oven to 200°C (400°F, gas mark 6). Trim any fat off the lamb and cut the meat into 2.5 cm (1 in) cubes. In a large flameproof casserole, dry-fry the lamb over a medium-high heat for 6-8 minutes until it is lightly browned, stirring frequently.
Add the carrots, celery and leeks and cook for a further 3-4 minutes, stirring occasionally.
Pour in the cider and stock, add salt and pepper to taste and bring to the boil. Cover and simmer over a low heat for 20-25 minutes until the vegetables are tender.
Meanwhile, make the topping. Sift the flour into a bowl and stir in the parsley and sage and a good sprinkling of salt and pepper. Stir in the fromage frais and bring the mixture together with your fingertips to make a fairly firm dough. If it is too dry, add 1-2 teaspoons of skimmed milk. Roll out the dough to about 1.5 cm (½ in) thick and cut it into 18-20 triangles.
Add the peas and bouquet garni to the casserole and arrange the triangles on top, covering the surface. Transfer the casserole to the oven and cook for 25-30 minutes until the topping is well risen and golden brown.