Pre-heat oven at 350ºF (180ºC).
Grease a 2-2½ quart baking dish.
Peel, core and slice the apples and place them in the baking dish. Add the lemon juice and mix well.
Mix the rest of the ingredients for the filling in a medium bowl and pour them over the apples. Toss the apples to make sure they're well coated.
In a separate medium bowl mix the flour, oats, brown sugar, white sugar, cinnamon and salt. Add the cold butter and mix with a pastry blender, fork or even your hands until it resembles a sandy, crumbly texture. Fold in the walnuts and sprinkle the topping all over the apples.
Bake for 45 minutes to 1 hour or until it's golden brown on top. Serve warm, either alone or with a side of ice cream.