Put oven rack in middle position and preheat oven to 375°F. Lightly oil a large shallow baking pan with vegetable oil. Arrange rhubarb in 1 layer in pan and sift confectioners sugar evenly over top. Bake, stirring occasionally, until rhubarb is very tender, 25 to 30 minutes. Transfer rhubarb to a food processor and puree until smooth. Force through a medium-mesh sieve into a bowl, discarding solids, and cool completely.