In a heavy-bottomed casserole, melt the butter over medium heat. Add onion, leek and shallots. Make them come back for 5 to 7 minutes, until they are melted, but without scorching them.
Add the pumpkin, pears and carrots. Stir until the pumpkin begins to soften. Add broth and thyme.
Cover and simmer 30 min., Until vegetables are cooked.
Remove the thyme. Pass the soup in the potato masher. Put it back in the pot and add the milk-cream mixture. Season and let heat for 5 minutes.
Transfer the soup to hollowed and golden bowls or pumpkins. Decorate with sage leaves, thyme, and a few spoonfuls of sour cream. Arrange the remaining cream in a saucier: each of the guests will be able to add it in its portion.