Makes 1 (9½-inch) pie

In a large bowl, or in the bowl of a food processor, combine flour, salt, and sugar. Cut butter into ½-inch pieces. Using a pastry blender or food processor, add the butter to the flour and mix to a coarse meal. Add the cold water and mix until dough forms. Form into a ball, cover with plastic wrap, and let the dough rest at least one hour in the refrigerator, or preferably overnight. Roll out and fit into a 9½-inch pie plate. Crimp edges and refrigerate 20 minutes. Fill with foil and pie weights and bake at 375°F until crust is set, about 15 minutes. Remove weights and bake until light brown.

In a heavy saucepan, mix rhubarb, strawberries, sugar, cornstarch, cinnamon, and nutmeg. Let stand 20 to 30 minutes. Bring to a boil, stirring frequently. Reduce heat and simmer until thickened, about 3 to 5 minutes. Pour filling into crust.

In a bowl, combine oats, flour, sugar, and cinnamon. Add the butter and cut it into the mixture until crumbly. Spread the mixture over the pie to cover. Bake at 375°F for 30 to 35 minutes, until bubbly.

In a bowl, beat the cream and sugar together until the mixture starts to thicken. Add the vanilla and Grand Marnier and whip until soft peaks form. Serve dolloped on top of pie.