How to Make It
Prepare Tart Shell: Pulse flour and salt in a food processor until combined. Add cubed butter, and pulse 10 to 12 times or until mixture resembles coarse breadcrumbs. Add powdered sugar and 3 egg yolks, and pulse just until mixture comes together and pulls away from sides of bowl. Divide dough in half, and flatten each half into a disk. Wrap in plastic wrap, and chill 1 to 12 hours.
Preheat oven to 350°. Place dough disks on a lightly floured surface, and roll each into a 12-inch circle. Fit dough circles into 2 (9-inch) fluted tart pans with removable bottoms; trim edges. Line tart shells with parchment paper, and fill with pie weights or dried beans. Bake at 350° for 20 minutes or until edges are light brown. Remove weights and parchment paper, and bake 5 to 10 minutes more or until light golden. Cool on a wire rack while preparing Filling.
Prepare Filling: Stir together lemon zest and next 4 ingredients in a large saucepan. Cook over low heat, whisking constantly, 3 minutes or until eggs have broken up and sugar has dissolved.
Whisk in 3/4 cup softened butter. (Eggs will start to cook, and mixture will coat the back of a spoon.) Whisk in remaining butter, and cook, whisking constantly, about 22 minutes or until mixture becomes very thick. (Continue whisking throughout the cooking process to prevent mixture from curdling.) Remove from heat, and pour through a fine wire-mesh strainer into a bowl, discarding solids. Whisk 10 minutes or until mixture has cooled slightly.
Increase oven temperature to 450°. Spoon lemon filling into tart shells, and bake 5 to 8 minutes or until top is set.
Prepare Meringue: Pour water to a depth of 1 1/2 inches in a small saucepan; bring to a boil over medium-high heat. Reduce heat to medium-low, and maintain at a simmer. Stir together egg whites and 2 cups granulated sugar in bowl of a heavy-duty electric stand mixer. Place bowl over simmering water, and cook, whisking constantly, 3 minutes or until sugar has dissolved and mixture is hot.
Beat hot mixture at medium-high speed with stand mixer, using whisk attachment, about 2 minutes. Increase speed to high, and beat 4 to 6 minutes or until stiff, glossy peaks form. Spoon meringue over tarts. Brown meringue using a kitchen torch, holding torch 2 inches from meringue and moving torch back and forth. (If you do not have a torch, preheat broiler with oven rack 8 inches from heat; broil 30 to 45 seconds or until golden.)