1To prepare crust: Combine ¾ cup nuts (or oats), whole-wheat flour, all-purpose flour, sugar and salt in a food processor; pulse until the nuts are finely ground. Add butter; pulse until well incorporated.
2Whisk egg, oil, 1 teaspoon vanilla and almond extract in a small bowl. With the motor running, add the mixture to the food processor. Process, then pulse, scraping down the sides, if necessary, until the mixture begins to clump, 30 to 45 seconds (it will look crumbly). Measure out ½ cup of the mixture and combine in a bowl with the remaining ¼ cup chopped nuts (or oats). Set aside for the topping.
3Preheat oven to 400°F. Generously coat a 9-by-13-inch baking dish with cooking spray.
4To prepare fruit filling & assemble bars: Combine 4 cups chopped fruit, orange juice, sugar and cornstarch in a large saucepan. Bring to a simmer over medium heat, stirring constantly, until the mixture is very thick, 4 to 5 minutes. (It may take up to 10 minutes to get a thick result if you start with frozen fruit.) Stir in the remaining 2 cups fruit and vanilla.
5Transfer the dough to the prepared baking dish. Spread evenly and press firmly into the bottom to form a crust. Spread the fruit filling over the crust. Sprinkle the reserved topping over the filling.
6Bake the bars for 15 minutes. Reduce oven temperature to 350 degrees and bake until the crust and topping are lightly brown, 25 to 30 minutes more. Let cool completely before cutting into bars, at least 1½ hours.
Make Ahead Tip: Cover and refrigerate the crust and topping (Steps 1-2) for up to 1 day. Cover or individually wrap and refrigerate the cooled bars for up to 5 days.
Note: Whole-wheat pastry flour is milled from soft wheat. It contains less gluten than regular whole-wheat flour and helps ensure a tender result in delicate baked goods while providing the nutritional benefits of whole grains. Find it at large supermarkets and natural-foods stores. Store in an airtight container in the freezer.
Easy cleanup: Dessert pans can be a headache to clean. Skip the soaking and scrubbing by lining your pan with parchment paper before you bake.