Heat oven to 400°F. Lightly spray an 8x8in glass baking dish with cooking spray.

In a large bowl, mix the oats, 1/2 cup flour and brown sugar. Cut in the butter.

If you do not have a pastry blender to do this, pull two butter knives through ingredients in opposite directions, until mixture is crumbly.

Stir in the walnuts and set aside.

In another bowl, stir together all the filling ingredients until mixed.

Pour the filling mixture into the glass baking dish, and sprinkle the entire bowl of topping mixture on top evenly.

Bake for 30-35 minutes. Mixture will be bubbly around the edges and topping will be golden brown.

Remove from oven and allow to cool for about 30-60 minutes.

Serve warm, enjoy!