Preheat oven to 350°F. 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 three 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.
Meanwhile, make Apple Cider Sauce.
To serve, cut into 8 pieces and place on dessert plates. Top with ice cream (or frozen yogurt) and drizzle with cider sauce.
Recipe: Apple Cider Sauce
Combine cornstarch and water in separate bowl. Slowly add cornstarch mix to boiling apple cider, stirring constantly until the sauce thickens slightly.