Preheat the oven to 375°F. Grease a 9x13-inch metal or glass pan with cooking spray or butter and set aside.

In a medium bowl, mix together the flour, rolled oats, brown sugar, cinnamon and salt. Whisk together until combined. With a pastry cutter or a fork, cut in the butter until the mixture resembles pea-sized crumbs. Place in the refrigerator while you prepare the apple filling.

Core, peel, and slice the apples into ¼ -inch to ½ -inch slices until you reach 6 cups of sliced apples, weighing 650 grams. Place in a medium-sized bowl and immediately drizzle with the lemon juice, tossing until evenly coated.

In a separate small bowl, mix together the flour, sugars, salt and spices (adjust to your preference). Pour dry mixture over the apples and toss to coat, mixing until evenly distributed amongst the apples.

Pour the apple mixture directly into the greased pan. Sprinkle the streusel evenly over the apples.

Bake for 30-35 minutes, or until the apples feel tender when pierced with a fork, and the topping is golden brown. Let the apple crisp cool for 20 minutes before serving. Serve warm with vanilla ice cream on top.