Preheat oven to 400 degrees F. Grease an 8x8 or 9x9 (or equivalent) baking dish with 1 tablespoon of the butter.
Mix the diced apples in a large bowl with 1 tablespoon of the cinnamon, 2 tablespoons of the brown sugar, and the lemon juice. Spread evenly in the buttered baking dish.
To make the topping, add the oats (1 cup), flour (1/2 cup), the rest of the brown sugar, and the remaining 1 tablespoon of cinnamon to a food processor. Cut the remaining 7 tablespoons of butter into small pieces and add on top. Pulse the food processor about 8-10 times, or until small, pea-sized chunks of butter form and the rest of the mixture has combined well. Alternatively, you can use your hands to do this in a bowl or a pastry cutter.
Spread the topping on top of the apples in the baking dish.
Sprinkle turbinado sugar (1 tablespoon) evenly on top, if using. At this point, you can cover with foil and refrigerate for up to 48 hours before baking, if you want.
Bake for 30-40 minutes, until browned on top and bubbly on the edges. Serve hot, warm, or at room temperature, with a scoop of vanilla ice cream on top, if you prefer.
If using unsalted butter, add 1/4 teaspoon of kosher salt to the topping mixture.
Instead of all-purpose flour, you can make it healthier by using whole wheat flour, or for a gluten-free version, you can use 1/2 cup more oats and pulse in your food processor a little more to make sure more oats are ground up.
You can keep this apple crisp out at room temperature for a few hours, but I recommend refrigerating it beyond that. Just pop it in the microwave or stick it back in the oven to reheat.
I used fuji apples for this because they are naturally sweet, which meant I could use less added sugar. If you use a more tart apple, like granny smith, I recommend using only 1 tablespoon of lemon juice and adding 2-4 more tablespoons of sugar to the recipe.
The provided nutrition information does not include any added sodium from seasoning to taste, any optional ingredients, and it does not take brands into account. Feel free to calculate it yourself using this calculator or by adding the recipe to Yummly.