Preheat the oven to 350 degrees F. Line your muffin tins with paper cups.
Whiz the squash in a food processor until finely chopped. Add the sugar, and crack in the eggs. Add a pinch of salt, the flour, baking powder, walnuts, cinnamon and olive oil and whiz together until well beaten. You may need to pause the machine at some point to scrape the mix down the sides with a rubber spatula. Try not to overdo it with the mixing - you want to just combine everything and no more.
Fill the paper cups with the muffin mixture. Bake in the preheated oven for 20 to 25 minutes. Check to see whether they are cooked properly by sticking a wooden skewer or a knife right into one of the muffins - if it comes out clean, they're done. If it's a bit sticky, pop them back into the oven for a little longer. Remove from the oven and leave the muffins to cool on a wire rack.
As soon as the muffins are in the oven, make your runny frosted topping. Place most of the clementine zest, all the lemon zest and the lemon juice in a bowl. Add the sour cream, icing sugar and vanilla seeds and mix well. Taste and have a think about it - adjust the amount of lemon juice or icing sugar to balance the sweet and sour. Put into the fridge until your muffins have cooled down, then spoon the topping onto the muffins.
Serve on a lovely plate (on a cake stand if you're feeling elegant, or on a rustic slab if you're more of a hunter-gatherer type!), with the rest of the clementine zest sprinkled over. For an interesting flavor and look, a few dried lavender flowers or rose petals are fantastic.
Recipe excerpted from Jamie At Home by Jamie Oliver. copyright (c) 2008. Published in the U.S. by Hyperion. All Rights Reserved. www.jamieoliver.com