Preheat oven to 375-degrees. Coat a deep 9 x 9-inch baking dish (or similar) with canola cooking spray.

Toast the walnuts by spreading them on a nonstick frying pan and lightly browning over medium heat until fragrant (about 4 minutes), turning often. Chop the nuts to medium-fine.

To make the topping, combine nuts, oats, 1/2 cup of whole wheat flour, 2 tablespoons brown sugar, and cinnamon in the bowl of an electric mixer. Drizzle the melted butter (or canola oil and a pinch of salt) and maple syrup over the top and blend on low speed until crumbly.

For the filling, add the apples, berries and water or fruit juice to a large bowl. Gently stir in 3 tablespoons sugar and 3 tablespoons whole wheat flour.

Spoon the mixture into the prepared baking dish then spoon the topping over the fruit, pressing down lightly.

Place dish on a baking sheet to catch the overflow.  Bake on the center rack until the topping is golden and the juices have thickened slightly (about 35 minutes). Serve warm with light vanilla ice cream if desired!