Preheat oven to 200°C (180°C fan assisted)/400°F/Gas 6.
Lay pumpkin on a baking sheet, cover with foil and cook in oven for 15- 20 minutes until soft. Meanwhile roll out pastry and use to line flan tin. Line with baking paper and fill with baking beans and bake blind for 15 minutes until just cooked but not too brown.
When the pumpkin is cooked, place in a food processor with sugar, cream, eggs, cinnamon and vanilla and whizz together until well mixed. Alternatively you can mix them all together by hand with a wire whisk.
Carefully pour filling into pre baked pastry case, reduce oven to180°C Gas M 4 and bake for approx. 40 minutes until set. Allow to cool
Whilst pie is cooking melt the butter and stir in sugar and chopped nuts. After 30 minutes baking time, remove pie from oven, scatter over the nut mixture, spreading lightly with a palette knife and return to oven for final 10 minutes.
Cool before serving with maple syrup and crème fraiche.