A cross between traditional apple crisp and more cake-like apple crumb bars, this dessert will meet your cravings for a luscious treat. A tart, firm apple like a Granny Smith provides the best combination of taste and texture, but the recipe can be made with whatever fruit you have on hand. Pears or berries would also work wonderfully in this simple, flexible recipe.
Preheat oven to 350 degrees. Coat a 9-inch square or 7-by-11-inch baking pan with cooking spray. Whisk oats, whole-wheat flour, all-purpose flour, brown sugar, lemon zest, baking powder, salt, cinnamon, and nutmeg in a large bowl. Work in the oil and apple-juice concentrate with a fork or your fingers until the mixture resembles coarse crumbs.
Press 2 cups of the oat mixture firmly into the bottom of the prepared pan. Arrange apple slices over the crust in 3 overlapping rows. Mix the walnuts into the remaining oat mixture. Sprinkle over the apples and pat into an even layer.
Bake until the top is golden and the apples are tender when pierced, 30 to 35 minutes. Cool completely on a wire rack. To serve, cut into 8 pieces and place on dessert plates. The squares are best served the day they are made.