Blend the flour, sugar, and salt together in a bowl. Work butter into the flour mixture and add egg yolk. Pat crust into an 8- or 9-inch pie pan.
Filling
Place chopped rhubarb into the bottom of pie crust. In a bowl, mix the eggs, sugar, sour cream (or buttermilk), and orange zest. Pour this mixture over the rhubarb and bake at 375 degrees for approx. 50 minutes, until the filling is firm and the fruit is tender.