In a food processor, pulse flour, sugar, and salt. Add butter; pulse until mixture resembles coarse meal. Add 2 tablespoons ice water; process until mixture just comes together when squeezed (add more water if necessary). Add oats; pulse to combine.
Press dough onto bottom and 1 inch up sides of a 9-inch springform pan. Bake until golden, about 25 minutes. Remove sides of pan; cool crust.
Whisk cooled pastry cream until smooth. Spread evenly in crust; arrange grapes on top; press in.
Chill at least 2 hours and up to 1 day. Just before serving, dust with confectioners' sugar.