How to Make It
Pulse first 4 ingredients in a food processor until blended. Add cold butter, and pulse 5 or 6 times or just until mixture resembles coarse meal. Stir together egg yolks and ice-cold water. With processor running, pour yolk mixture through food chute, and process just until mixture forms a ball and pulls away from sides of bowl. Wrap dough in plastic wrap; chill 1 hour.
Meanwhile, melt 3 Tbsp. butter in a 1-qt. saucepan over medium heat. Add rhubarb, and sauté 3 minutes. Stir in strawberries and 2/3 cup granulated sugar; cook, stirring constantly and crushing fruit with spoon, 5 minutes.
Stir together 1 Tbsp. flour and 2 Tbsp. lemon juice until smooth. Stir juice mixture into rhubarb mixture; bring to a boil. Cook, stirring often, 2 minutes or until thick. Remove from heat, and transfer mixture to a small bowl. Cover and chill 30 minutes.
Preheat oven to 350°. Unwrap dough; roll to 1/8-inch thickness on a floured surface. Cut dough into 48 rectangles, using a 2 1/2- x 3-inch cutter and rerolling scraps. Place half of dough rectangles 2 inches apart on parchment paper-lined baking sheets. Top each rectangle with about 1 Tbsp. strawberry mixture. Dampen edges of dough with water, and top with remaining dough rectangles, pressing edges to seal.
Stir together egg and 2 Tbsp. milk, and brush over tops of tarts. Cut a small "X" in top of each tartlet for steam to escape.
Bake tartlets at 350° for 30 to 35 minutes or until golden. Transfer tartlets to a wire rack; cool 15 minutes. Whisk together powdered sugar and remaining 1 Tbsp. lemon juice and 1 1/2 tsp. milk; drizzle over tartlets.