Heat 1 tablespoon of olive and the butter in a pan over medium-low heat. Sweat onions for about 10 minutes until soft and translucent.
Add chopped sage and garlic and sauté for one minute.
Reserve a handful of chestnuts and add the rest to the pan with the vegetable broth/stock. Finely chop reserved chesnuts for serving
Season with salt and pepper as required and increase the heat to simmer for 15 minutes, stirring occasionally.
Remove from heat and use handhled blender to blend to a smooth consistency.
Stir in crème fraîche and return to a low heat to warm through.
Heat another tablespoon of olive oil in a small fry pan and sauté some whole sage leaves for a few seconds until crisp. Drain on paper towel.
Meanwhile make crostini. Lay ciabatti slices onto a greased baking tray and drizzle with olive oil. Bake at 200C/400F/Gas 6 for 8-10 minutes until golden.
Cut garlic clove in half and use the cut side to rub over the toasted bread. Sprinkle over chopped herbs.
To serve soup, ladle into warmed bowls. Sprinkle chopped chestnuts, and top with a crostini and fried sage leaf.