Grease 20 holes in two 12-hole patty pans or mini muffin tins.
To make the pastry, sift the flour and ¼teaspoon salt into a bowl. Add the butter and rub in with your fingertips until the mixture resembles fine breadcrumbs. Make a well in the centre. Crush the peppercorns with a knife and chop finely. Add to the flour with the egg yolk, mustard and 2teaspoons chilled water. Mix with a flat-bladed knife, using a cutting action, until the mixture comes together in beads. Turn onto a lightly floured work surface and press together into a ball. Cover with plastic wrap and refrigerate for 30 minutes.
Preheat the oven to 200°C (400°F/Gas 6). Roll the dough out on a floured surface to 3mm (⅛inch). Cut out 20 rounds with an 8cm (3¼inch) cutter. Put in the patty pans and prick with a fork. Bake for 8 minutes, or until golden.
To make the filling, heat the oil in a heavy-based saucepan. Cook the onion and garlic over low heat for 30 minutes, or until the onion is very soft and beginning to brown. Increase the heat to medium, add the sugar and vinegar and cook, stirring, until most of the liquid has evaporated and the onion is glossy. Stir in the raisins.
Spread a little olive paste into the base of each pastry case. Spoon the onion mixture over it and crumble the feta on top. Serve warm.