Arrange a rack in the middle of the oven and preheat it to 350° F (177° C).
Make the streusel topping by combining flour, oats, brown sugar, and salt in a bowl. Add melted butter and vanilla extract.
Mix them together and refrigerate while you prepare the apples.
Peel the apples and remove the core. Cut them evenly into chunks with about ¾-inch thickness. Squeeze half of the lemon over them.
Add sugar, cornstarch, and cinnamon. Toss them together.
Transfer the apples to your baking dish.
Take the streusel topping from the fridge. Distribute them evenly over the apples.
Bake for 50-60 minutes until the top is golden brown and juices are bubbling around the edges.
Enjoy warm with a scoop of vanilla ice cream or whipped cream.
Notes
Apples: Use apples that keep their shape during baking and don't turn mushy. Some examples are Jonagold, Honeycrisp, Pink Lady (or Cripps Pink), Fuji, Gala, Braeburn, Cortland, and Granny Smith apples. I used a combination of sweet Golden Delicious and tart Empire apples in this recipe.
Oats: Use old-fashioned oats (rolled oats or whole oats) for a chewy, crunchy texture. Do not use steel-cut oats; instant oats will disintegrate into the mixture.
Optional ingredient: Walnuts or pecans will add more crunch to the topping.