Make the lemon curd Bring 2 inches of water in a 3-quart saucepan to a simmer over medium heat. In a stainless-steel bowl that fits snugly in the saucepan without touching the water, combine the sugar, lemon juice, lemon zest, and rosemary. Set the bowl over the saucepan, whisk in the egg yolks, and continue to whisk until the mixture thickens and registers 160°F on an instant-read thermometer, 5 to 10 minutes.
Turn off the heat, leaving the bowl over the water. Add the salt and then whisk in one piece of butter at a time, whisking until smooth between each addition. Strain through a fine sieve into a medium bowl and cover with plastic wrap, pressing it directly on the surface of the curd. Refrigerate until well chilled, 3 hours or up to 2 days.
Make the shortbread dough Put the flour, confectioners’ sugar, pine nuts, and salt in a stand mixer fitted with the paddle attachment, and mix on low speed to combine. Add the butter and continue to mix on low speed until the dough is smooth, about 1 minute. Divide the dough in thirds, wrap each piece in plastic, and press into flat disks. Refrigerate for at least 2 hours or overnight.
Bake the shells in two batches Spray twenty-five 2-inch flared or fluted tartlet molds with cooking spray. Working with one round of the shortbread dough at a time, roll the dough between two pieces of parchment to a 1/8-inch thickness. Using a 2-1/2-inch round cutter, cut the dough into circles—you’ll need 25 circles for the first batch. Gather any scraps and refrigerate for re-rolling. Using a metal spatula, transfer the rounds to the tartlet pans and press the dough into the pans. Trim the tart dough so that it’s flush with the pan by pressing along the rim of the pan with your thumb. Prick each tartlet shell several times with a fork, arrange on a rimmed baking sheet, and refrigerate until chilled, about 30 minutes.
Meanwhile, position a rack in the center of the oven and heat the oven to 350°F.
Bake the tartlet shells until they’re deep golden-brown, 10 to 15 minutes. Transfer the shells to a rack to cool. Remove the shells from the pans and store in a sealed container at room temperature for up to 2 days.
Repeat rolling and cutting the remaining dough and scraps, which you can re-roll once, to yield 25 more circles. Bake, cool, and store as above.
Make the garnish Heat the oil in an 8-inch skillet over medium heat. When the oil shimmers, add the pine nuts. Toss the nuts until browned, 30 seconds to 1 minute. With a small slotted spoon, transfer the nuts to a paper-towel-lined plate. Sprinkle with a pinch of salt, and cool. Store in an airtight container for up to 2 days.
Assemble the tartlets Put the lemon curd in a pastry bag fitted with a 1/2-inch plain tip (Ateco #806). Pipe the curd into the tartlet shells and top each with 3 pine nuts. (Reserve any remaining curd for another use.) The tartlets may be assembled up to 4 hours ahead.
