This is a recipe I got from a Taste of Home magazine 1997.
4 cups sliced fresh or frozen rhubarb, thawed
4 cups boiling water
1 1/2 cups sugar
3 tbsp all-purpose flour
1 tsp quick-cooking tapioca
1 egg
2 tsp cold water
Pastry for a double 9-inch pie
1 tbsp 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 and toss to coat. Let stand for 15 minutes. In a small bowl, whisk together egg and cold water. Stir into the rhubarb mixture.
Preheat oven to 400-degrees. On a lightly floured surface, roll one half of the pastry dough to a 1/8-inch circle and transfer to a 9-inch pie plate. Trim pastry even with the rim. Add the filling and dot with the butter. Roll out remaining pastry dough and place over the filling. Trim, seal and flute edge. Cut air slits in the top. Bake for 15 minutes.
Reduce oven to 350-degrees. Bake 40 to 50 minutes or until crust is golden brown and filling is bubbly. Cool on a wire rack.
Note: If using frozen rhubarb, measure while frozen, then thaw. Drain in a colander but do not press liquid out.
Pastry for a 9-inch pie: Combine 2 1/2 cups of flour and 1/2 tsp salt. Cut in 1 cup cold butter until crumbly. Gradually add 1/3 to 2/3s ice water, tossing with a fork until dough holds together when pressed. Wrap in plastic wrap and refrigerate 1 hour before rolling out.