4 cups sliced fresh or frozen rhubarb, thawed
4 cups boiling water
1-1/2 cups sugar
3 tablespoons all-purpose flour
1 teaspoon quick-cooking tapioca
1 large egg
2 teaspoons cold water
Pastry for double-crust pie (9 inches)
1 tablespoon butter
Place rhubarb in a colander; pour boiling water over rhubarb and allow to drain. In a large bowl, mix sugar, flour and tapioca. Add drained rhubarb; toss to coat. Let stand 15 minutes. In a small bowl, whisk egg and cold water; stir into rhubarb mixture.
Preheat oven to 400°. On a lightly floured surface, roll one half of the pastry dough to a 1/8-in.-thick circle; transfer to a 9-in. pie plate. Trim pastry 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. Bake 15 minutes.
Reduce oven setting to 350°. Bake 40-50 minutes longer or until crust is golden brown and filling is bubbly. Cool on a wire rack.