Rinse goose legs, pat dry, rub with sugar and salt and refrigerate overnight. The next day, boil legs in a large pot, cover and let simmer over low heat for about 2 hours. Skim off foam. Remove legs and let cool. Strain goose stock through a fine sieve and into a saucepan. Boil again over high heat, measure 80 ml (approximately 1/3 cup) and set aside. Allow stock to cool. Remove meat from the bone, taking care to remove the skin, tendons and cartilage.
Using a mortar and pestle, coarsely crush juniper berries, allspice and peppercorns. Heat lard in a pan and cook juniper, allspice, pepper and bay leaf. Simmer for 10 minutes over low heat. Pour mixture through a sieve and into a bowl. Zest lemon. Rinse sage, shake dry, pluck the leaves and mince.
Preheat the oven to 200°C (approximately 400°F).
Mix meat with juniper berry mixture, lemon zest, chopped sage and the lard. Allow to cool slightly. Stir in eggs.
Roll out puff pastry and line a 26 cm (approximately 10 inches) tart tin with dough.
Pour meat filling into the pastry-lined tart tin. Remove excess dough with a sharp knife.
Bake in preheated oven for 30-40 minutes.
Rinse apples, remove seeds and slice. Remove tart from oven when 15 minutes of cooking time remains. Top with apple slices, sage and sea salt. Return to oven and cook until done.
Remove from oven and serve warm or cold.