Add the apples, lemon juice, and vanilla extract to a mixing bowl. Then stir in the cinnamon and sugar. Set the bowl aside.
Add flour, sugar, walnuts, oats, and cinnamon to a separate bowl and mix well. Stir in the melted butter and mix until crumbly.
Pour the filling into a baking dish and use a large wooden spoon or spatula to make a even layer. Add the 2 Tbsp of water to the pan. Pour the topping over top and shake the pan a bit to even it out.
Bake at 375°F for about 45 minutes. Then remove from the oven and let cool for 10–15 minutes.
Serve with vanilla ice cream or whipped cream.