1.First, make the pastry. Place the flour and butter in a food processor and blend until mixture resembles breadcrumbs (you can rub between your fingertips if you don’t have a food processor).
2.Add the egg yolk and 3 tbsp cold water then blend until the mixture forms a ball. Turn out and knead lightly.
3.Roll out on a lightly floured surface and use to line a 25cm round fluted flan tin. Prick the base and sides with a fork. Cover and chill for 1 hour.
4.Line the pastry case with parchment paper and baking beans and bake blind in a preheated oven Gas 5, 190 C/ Fan 170 C for 10 minutes.
5.Remove the beans and paper then bake for a further 5 minutes before removing from the oven. Reduce the oven temperature to Gas 4, 180 C/ Fan 160 C.
6.In a pan of boiling water simmer the cauliflower for 5 mins until it starts to soften then drain and set aside.
7.Gently fry the onion in Yeo Valley butter for 3-4 mins until soft.
8.Using a knife, trim the excess pastry from the edge of the case then tip the cauliflower and ham into the pastry shell so they are fairly evenly distributed.
9.In a bowl combine the eggs, crème fraiche, chives and half the cheese and season to taste, then pour the egg mixture into the pastry case and top with the remaining cheese.
10.Cook in the oven for 30 mins until the top is golden brown with just a slight wobble in the centre.
Serve warm or cold.