Pre-heat oven to 350 degrees F.
Toss sliced apples in flour, cinnamon and lemon juice in a large bowl.
Place apples into a square baking dish (8 x 8 inches).
In the same bowl, add the sliced butter, oats, brown sugar, salt and cinnamon.
Using 2 knifes "cut" through the mixture, combining the ingredients together until it becomes a crumbly texture. Add walnuts and stir.
Add the oat crisp topping overtop the apples, pressing down gently.
Bake for 30 - 35 minutes in the oven or until the crisp is hot throughout.
Serve warm.