Preheat oven to 375. Butter a medium-sized shallow casserole dish (a 9×13 dish).
Peel apples and then chop them into 1/2-inch pieces right into the buttered dish. Add the lemon juice to the apples and gently toss to coat the apples. Add the vanilla, cinnamon, brown sugar, and cornstarch and gently mix until distributed throughout the apples.
For the topping, stir together flour and brown sugar, breaking up any lumps. Cut in butter. Stir in pecans or any nuts you choose (can leave them out as well). Spoon about 1/3 on the apples and mix in throughout the mixture, then sprinkle the rest over the top.
Put in the preheated oven at 375 for about 35-45 minutes or until the topping is golden brown.
Remove from oven and let cool so mixture comes together nicely. Serve with vanilla ice cream or whipped topping. Mmm, Mmm.
Notes
Use a shallow baking dish.
I highly recommend using a mix of Granny Smith and Golden Delicious apples.
Cold butter works best for the streusel topping.
If you want to use chopped toasted pecans or walnuts in the topping, decrease the flour by 1/2 cup and use 1 cup of chopped nuts.
Need to be gluten free? No problem. You can substitute almond flour for the regular flour.