Make the lime curd: Place a fine-mesh sieve over a medium bowl and position it near the stovetop. Bring a pot of shallow water to a gentle simmer.
In a metal bowl, whisk together the egg yolks, granulated sugar, and lime zest until thick and pale, about 2 minutes. Whisk in the lime juice and salt. Set the bowl over the simmering water. The bottom of the bowl should hover an inch or two above the water’s surface. Whisking constantly, cook until the curd coats the back of a spoon, 5 to 7 minutes (or until an instant-read thermometer registers 165 to 170°F/75 to 77 ̊C). Remove the curd from the heat and press it through the sieve into a clean bowl. Add the butter to the curd; let it stand 1 minute. Stir to combine. If the butter doesn’t totally melt, let the curd rest another 1 to 2 minutes and stir again.
Place a piece of plastic wrap directly onto the surface of the curd. Let it cool to room temperature, then refrigerate until chilled, at least 4 hours. The curd may be made up to 2 days in advance.
Make the dough: Place the flour, confectioners’ sugar, salt, and poppy seeds in the bowl of a food processor and pulse to combine. Add the cold butter and pulse until it is in pea-sized pieces. Add the egg and pulse just until the dough comes together. If the dough has dry patches, add 1 tablespoon ice water and pulse again. Transfer the dough to a clean work surface and press it into a round. Wrap it in plastic and refrigerate for at least 2 hours or overnight.
Make the tart: On a lightly floured surface, roll the dough into a 1⁄4-inch-thick (6-mm-thick) round. Use the tablespoon of soft butter to grease a 9-inch (23-cm) fluted tart pan with a re- movable bottom. Press the dough into the bottom and sides of the pan. Trim the dough, leaving a 1⁄2-inch (12-mm) overhang. Fold the overhang underneath the dough and pinch together to reinforce the sides. Using a fork, pierce the bottom of the crust all over. Freeze for 1 hour.
Preheat the oven to 375oF (190 ̊C). Bake the crust until golden and crisp, 25 to 30 minutes. Let it cool completely.
Pour the lime curd into the tart shell. Refrigerate, uncovered, for at least 2 hours. When ready to serve, top with the raspberries and currants, if using.
