Step #1 To make the Sweet-Tart Dough: Using an electric mixer fitted with the paddle attachment, beat the butter until soft & very smooth.
Step #2 Add the confectioners' sugar, & beat until mixed.
Step #3 Add the flour & salt.
Step #4 With the mixer running at lowish speed, beat the dough until it becomes crumbly.
Step #5 Add the egg, & continue mixing until the dough just comes together (do not overwork it).
Step #6 Turn out the dough onto a work surface, & gather into a ball.
Step #7 Cut in half, & shape each half into a disk.
Step #8 Wrap both pieces of the dough in plastic.
Step #9 put in the fridge at least 1 hr.
Step #10 Lightly butter the insides of two 8- by 3/4-inch round tart rings.
Step #11 Place the rings on a baking sheet lined with parchment paper.
Step #12 Working on a lightly floured work surface, roll one disk of the dough to an 1/8-inch thickness.
Step #13 Fit the dough into a tart ring, pressing lightly against the bottom & up the sides.
Step #14 Run your rolling pin against the rim of the circle, & cut off the excess.
Step #15 Repeat with second disk of dough & tart ring.
Step #16 Chill 30 mins.
Step #17 Heat oven to 350 degrees with rack in center.
Step #18 Line each tart shell with a circle of parchment paper, & fill shells with pie weights or dried beans.
Step #19 Bake crusts 15 mins, remove the paper & beans, and bake just until the edges of the crusts are golden, 3 to 5 mins.
Step #20 Transfer baking sheet to a rack to cool.
Step #21 To make the Filling: Grate the zest of each lemon using the small holes of a box grater, & set aside.
Step #22 With a small knife, cut off enough of the top & bottom of each lemon, exposing the flesh.
Step #23 Stand the lemons on end, and carefully cut away the white pith & a tiny bit of the flesh from the entire surface of each lemon; the juicy sections of lemon should now be completely exposed.
Step #24 Lay the lemons on their sides, & cut each lemon crosswise into 1/2-inch-thick slices; remove seeds.
Step #25 Place lemon slices, whole eggs, egg yolk, granulated sugar, & heavy cream in the jar of a mixer, & puree until smooth.
Step #26 Strain the mixture into a bowl, & whisk in the reserved lemon zest.
Step #27 Lower the oven temperature to 300F degrees.
Step #28 Scatter the raspberries over the bottom of the two crusts.
Step #29 Pour the lemon filling into crusts.
Step #30 Bake tarts just until the filling has set in the centers, about 25 mins; the filling should not shimmy when the edges of the tarts are tapped lightly.