This is a luxe take on quiche Lorraine. Because of the depth of
the tart and the amount of filling, we've precooked the custard
mixture over a bain-marie to ensure it cooks through in the oven
without the tart burning.
For pastry, process butter, flour and a pinch of salt in a food processor until fine crumbs form. Add yolk and 60ml iced water, process until just combined. Form pastry into a disc, wrap in plastic wrap and refrigerate to rest (1 hour). Roll out pastry on a lightly floured surface to 5mm thick, then line a buttered and floured 24cm-diameter springform tin lined on the base with baking paper. Trim edges, prick base with a fork, refrigerate to rest (1 hour).
Preheat oven to 190C. Blind bake pastry until starting to turn golden (25-30 minutes), remove paper and weights and bake until cooked and light golden (10-15 minutes). Keep warm.
Meanwhile, heat oil in a frying pan over medium heat, add onion and chopped prosciutto, sauté until tender (5-10 minutes). Add thyme, season to taste, set aside.
Whisk eggs, sour cream and cream in a heatproof bowl until well combined. Season to taste, then stir continuously over a saucepan of simmering water until mixture coats the back of a spoon thickly (10-15 minutes). Stir in Gruyère and onion mixture, spoon into pastry case. Reduce oven to 175C and bake tart until set (30-40 minutes). Cool to room temperature in tin on a wire rack (2-3 hours).
Meanwhile, place prosciutto slices on an oven tray lined with baking paper and bake, turning occasionally, until crisp (10-15 minutes). Place on top of tart and serve.