The filling for these portable tarts packs an abundance of flavor: Sliced tomatoes are roasted until their flavors are concentrated and the tomatoes have shed much of their moisture (so the tarts won't get soggy). This is a great way to prepare tomatoes even if you don't make the tarts; store them in an airtight container in the refrigerator, cover in a slick of olive oil, then bring to room temperature and serve on crostini, burgers, or pasta.
Make the dough: Pulse together flour and salt in a food processor until combined. Add butter, and process until mixture resembles coarse meal, about 10 seconds. With machine running, add just enough ice water in a slow, steady stream until dough just comes together (no longer than 30 seconds). Divide dough in half, shape each piece into a square, and wrap in plastic. Refrigerate at least 1 hour or up to 2 days (or freeze up to 1 month; thaw overnight in refrigerator before using).
Roll out dough on a lightly floured surface to 1/8 inch thick. Using a paring knife, cut out twelve 4 1/2 inch squares, and fit into cups of a standard muffin tin, leaving an overhang. Chill until firm, about 30 minutes.
Make the filling: Preheat oven to 400ºF. Divide tomato and onion slices between two rimmed baking sheets. Drizzle with oil, season with salt and pepper, and roast, rotating sheets halfway through, until tomatoes begin to shrivel and onion slices are golden, about 30 minutes. Let cool on sheets, then transfer to a bowl.
Lower oven temperature to 375ºF (or preheat separate oven). Divide half the chopped oregano, olives and feta among the pastry shells. Top with tomato-onion mixture, dividing evenly. Sprinkle with remaining oregano, olives, and feta. Fold corners of dough toward centers. Brush with egg wash.
Bake pies until bubbling and top crusts are golden, 50 to 60 minutes. Let cool completely in tin on a wire rack before serving, with oregano sprigs tucked into pies, if desired.