Make the dough: MIx flour, sugar, and salt in a bowl. Using a dough blender, two forks, or your fingers, cut butter into flour mixture, forming pea-size crumbles. Whisk milk and 1 egg in a separate bowl and add to flour mixture; work dough until smooth but with visible flecks of butter. (Alternatively, pulse ingredients in a food processor.) Pat dough into a flat disk and wrap in plastic wrap; chill at least 1 hour before using.
Make the filling: Melt butter in a 2-qt. saucepan over medium. Cook strawberries and rhubarb until soft and jamlike, 6–8 minutes. Stir in sugar, vinegar, pepper, lemon juice, and salt; cook 3 minutes more. Let cool to room temperature, then cover and chill at least 30 minutes before using.
Assemble and bake the pies: On a lightly floured surface, roll dough 1⁄4″ thick. Trim edges to make a 10″x12″ rectangle. Cut dough into six 4″x5″ rectangles; gather dough scraps, re-roll, and cut one more rectangle. Place 2 tbsp. filling in center of each rectangle. Whisk remaining egg in a bowl and brush edges of rectangles; fold one short side of dough over the other, encasing the filling. Crimp edges using a fork. Transfer pies to parchment paper-lined baking sheets and chill 20 minutes. Beat remaining egg; cover and chill.
Heat oven to 400°. Using a fork, prick tops of pies; brush tops with the remaining beaten egg. Bake pies until golden, about 20 minutes; let cool slightly before serving