2 tablespoons finely chopped orange zest, (see Tip)
1 1/4 cups all-purpose flour
1 tablespoon sugar
1/4 teaspoon salt
1 tablespoon butter
1/4 cup canola oil
2-3 tablespoons ice water
To make the filling: Combine 6 cups rhubarb, sugar and orange zest in a wide heavy-bottomed saucepan. Bring to a boil, then reduce the heat to low and simmer gently. Cook, stirring often to prevent scorching, until the puree has reduced to about 1 2/3 cups, 40 to 50 minutes. Transfer the puree to a bowl and let cool to room temperature.
To make the crust: Position rack in lower third of oven and set a large baking sheet on it; preheat to 400 °F. Lightly oil a 10-inch removable-bottom tart pan or coat it with nonstick cooking spray; set aside.
Stir together flour, sugar and salt in a medium bowl. Melt butter over low heat in a small saucepan. Cook, swirling the pan, until the butter turns a nutty brown, about 30 seconds. Pour the butter into a small bowl; stir in oil. Using a fork, slowly stir the butter-oil mixture into the dry ingredients until the mixture is crumbly. Gradually stir in enough ice water so that the dough holds together and is not at all crumbly. Press the dough into a flattened disk.
Place two overlapping lengths of plastic wrap on the work surface. Set the dough in the center and cover with two more overlapping sheets of plastic wrap. With a rolling pin, roll the dough into a 14-inch circle. Remove the top sheets and invert the dough into the prepared tart pan. Gently press the dough into the pan. Remove the remaining plastic wrap. Fold in the overhanging edge of the crust to form a sturdy edge, patching any thin spots with scraps.
To assemble and bake the tart: Combine the raw and cooked rhubarb and spread in the crust. Place the tart pan on the preheated cookie sheet and bake for 35 to 40 minutes, or until the crust is golden and the filling bubbles at the edges. Let the tart cool to room temperature before serving.
Make Ahead Tip: Cover and refrigerate the puree (Step 1) for up to 2 days.
Tip: To remove citrus zest: Use a vegetable peeler to easily remove strips of the outer skin (zest), leaving the bitter white pith behind.