Rhubarb-Strawberry Tart with Crisp Oat Topping

For the crust:

1 1/4 cups all purpose flour, plus more for dusting
1/2 teaspoon coarse salt
1/2 teaspoon granulated sugar
1/2 stick (4 tablespoons) cold unsalted butter, cut into pieces
2 tablespoons ice water

For the topping:

1 cup all purpose flour
3/4 cup rolled oats
1/4 cup brown sugar
1/4 teaspoon ground cinnamon
1/2 teaspoon coarse salt
1 stick (1/2 cup) cold unsalted butter, cut into pieces

For the filling:

1 vanilla bean, halved lengthwise
1 pound rhubarb, sliced 1/4 inch thick (about 4 cups)
8 ounces strawberries. hulled and quartered
1 cup granulated sugar
1 teaspoon finely grated orange zest
3 tablespoons fresh orange juice
3 tablespoons all purpose flour
1 teaspoon coarse salt

For the crust: Pulse flour, salt, and sugar in food processor until combined. Add butter, and pulse until mixture resembles coarse crumbs with a few large pieces remaining. With the machine running, add the ice water in a slow stream and process until dough just holds together. (Dough should not be sticky).

Turn dough onto a lightly floured work surface and shape into a disk. Wrap in plastic and refrigerate for at least 30 minutes (or overnight). Dough can be frozen for up to 1 month; thaw overnight in the refrigerator.

For the topping: Combine flour, oats, sugar, cinnamon, and salt in a bowl. Cut in butter with a pastry blender or your fingers until large clumps form. Refrigerate, uncovered, for at least 15 minutes (or covered for up to 2 weeks).

Preheat oven to 350. On a lightly floured work surface, roll out dough to a 14 inch round about 1/8 inch thick. Transfer to a 9 by 1 2'4 inch tart ring set on a parchment lined baking sheet, pressing crust onto sheet and up sides of mold. Trim excess flush with rim. (Alternatively, press dough into bottom and up sides of a 9 inch springform pan. Using a knife, trim sides to be 1 3/4 inch high.) Freeze dough, uncovered, for 15 minutes.

Cut a 13 inch round of parchment, and place on top of crust. Fill with pie weights or dried beans. Bake until the edges are just golden, 10-15 minutes. Let crust cool completely on sheet on a wire rack.

For the filling: Using the tip of a paring knife, scrape seeds from vanilla bean into a medium bowl, reserving pod for another use. Add the remaining ingredients and stir until combined. Spread on cooled crust and sprinkle the chilled topping evenly over the filling.

Bake until topping is crisp and filling is bubbling, about 1 hour. Let cool on a wire rack for 30 minutes. Carefully remove tart ring, and let cool completely on sheet. Transfer to a serving plate.