To make the pastry, put the flour and butter in a food processor and whiz until the mixture resembles breadcrumbs. Put into a bowl, add a pinch of salt, the egg yolk and 7 teaspoons of cold water. Mix well and bring the dough together with your hands.Wrap and chill for 15 minutes.

To make the filling, melt the butter and oil together in a pan. Toss the onions in 1 tablespoon of lemon juice, then saute the onions for 15 minutes. Put to one side.

Divide the pastry into six pieces. Roll each piece on a floured board and use to line six 11 cm (4 1/4 in) loose-based tartlet tins. Prick the base and sides with a fork. Put on a lipped baking sheet, cover with clingfilm and freeze for 10 minutes.

Preheat the oven to 200°C/180°C fan oven/Gas Mark 6. Line tartlet tins with greaseproof paper and baking beans and bake for 10 to 15 minutes. Remove the paper and beans and return to the oven for 5 minutes or until dry in the centre.

Increase the oven temperature to 230°C/210°C fan oven/Gas Mark 8. Put half the onions in the pastry cases. Mix the creme fraiche and thyme together, season and spoon half over the onions. Add half the parmesan. Top with the rest of the onions, creme fraiche and parmesan. Bake for 5 to 10 minutes or until golden. Serve garnished with thyme sprigs.