In a small bowl, whisk egg, 4 tablespoons ice water and vinegar until blended. In a large bowl, mix flour and salt; cut in lard until crumbly. Gradually add egg mixture, tossing with a fork, until dough holds together when pressed. If mixture is too dry, slowly add additional ice water, 1 teaspoon at a time, just until dough comes together., Divide dough in half. Shape each into a disk; wrap and refrigerate 1 hour or overnight., Preheat oven to 400°. In a large bowl, mix sugar and tapioca. Add rhubarb and strawberries; toss to coat evenly. Let stand 15 minutes., On a lightly floured surface, roll half the dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim crust even with rim., Add filling; dot with butter. Roll remaining dough to a 1/8-in.-thick circle. Place over filling. Trim, seal and flute edge. Cut slits in top. Brush milk over top crust; sprinkle with coarse sugar. Place pie on a baking sheet; bake 20 minutes., Reduce oven setting to 350°. Bake 45-55 minutes longer or until crust is golden brown and filling is bubbly. Cool pie on a wire rack.