The ‘freeform tart’ is a great introduction to making and rolling out shortcrust pastry for a large tart. The onions slowly caramelise, creating a tantalising smell.
Start with the filling. Halve the onions lengthways, then cut them into thin, irregular slices. Heat the oil and butter in a large, deep frying pan. As the butter starts to sizzle, tip in the onions and stir to coat well. Fry over a medium heat for about 20–25 minutes, stirring only occasionally. When the onions are sticky and caramelised, remove them from the heat and season with salt and pepper.
To make the pastry, put the flour, butter and a pinch of salt in a large bowl. Rub in until the mixture looks like fine breadcrumbs. Gradually pour in just enough cold water (2–3 tablespoons) so the dough comes together and stir with a round-bladed knife to form a dough. Gently work together into a smooth ball with your hands, being careful not to over-handle it. (Or make the pastry in a food-processor.) Shape the dough into a thick disc, wrap in clingfilm and chill in the fridge for 15–20 minutes, until firm but not hard. Preheat the oven to 200℃/180℃ fan/400°F/Gas 6. Line a large baking sheet with baking paper.
Roll out the pastry on a lightly floured surface to a 25cm circle, about the thickness of a £1 coin. Transfer to the baking sheet by draping it over a rolling pin. Brush all round the edge with water and fold over the edge to create a rough rim that will contain the filling. Your freeform circle should now be about 23cm.
Now fill your tart. Spread the cooled onions over the bottom of the pastry right up to the rim. Beat the egg in a bowl, stir in the crème fraîche, Dijon mustard and season with pepper and a little salt. Tear the taleggio (or other cheese) slices in half and lay them over the onions. Pour the egg and crème fraîche mixture over the top. Tear the prosciutto slices into pieces and scatter them over the tart. Brush the pastry rim with a little milk.
Bake for about 25 minutes, or until the pastry is cooked and pale golden and the prosciutto crisp. Serve warm or at room temperature sprinkled with a few rocket leaves.
Photography: Amanda Heywood © Hodder & Stoughton