2Heat butter in microwave oven in a 4-cup microwave-safe measuring cup until melted but not hot, about 1 minute. Beat eggs, sugar, and flour into butter with a fork until smooth. Spread the rhubarb into the unbaked pie crust and pour the liquid ingredients over the rhubarb.
3Bake pie in the preheated oven until the filling is lightly browned, about 45 minutes.