Preheat oven to 350°F. Coat a 9-inch-square baking pan with cooking spray.
Combine flour, oats, brown sugar, lemon zest, baking powder, salt, cinnamon and nutmeg in a large bowl. Work in oil and apple juice concentrate with your fingers until coarse crumbs form.
Firmly press 2 cups of the oat mixture into the prepared pan. Arrange apples over the crust in three rows. Mix walnuts into the remaining oat mixture. Sprinkle the walnut mixture evenly over the apples and pat firmly into an even layer.
Bake until the top is golden brown and the apples are tender when pierced with a sharp knife, 30 to 35 minutes. Cool completely on a wire rack.