Preheat oven to 375 degrees.
Combine in bowl the first 4 ingredients.
Cut in the butter
Pat mixture over bottom and sides of a 9-inch square ovenproof dish, or 10-inch round pie plate. The mixture will seem dry – just get it spread evenly in the pan, not pressing down too hard.
Arrange rhubarb in pastry.
Sprinkle over sugar and cinnamon
Bake for 15 minutes.
Meanwhile, combine egg, sour cream and milk
Pour over rhubarb and bake 30-40 minutes longer, until the custard seems moderately solid when the pan is shaken gently. Custard will continue to set up as the pie cools.