Preheat the oven to 350 degrees.
Remove stem from pumpkin, slice in half and place on a lined baking sheet cut side down. Bake 45 minutes. Meanwhile, melt butter in a heavy-bottomed pot.
Add onion and leeks and saute until soft and translucent over low heat, stirring occasionally.
When pumpkin is done, remove from oven, scoop out flesh and discard skin.
Add pumpkin and chicken stock, bring to a boil and reduce to simmer. Cool 8 - 10 minutes.
Puree in a blender or food processor and return to stove. Add heavy cream and two tablespoons finely chopped sage leaves. Season to taste with salt and pepper.
Heat oil in small frying pan over medium heat. Add 12 whole sage leaves and fry until crispy. Serve soup hot, garnished with crispy sage leaves.