alex's slightly modified version of Food52's best apple crisp

click here for original recipe by Ella Quittner via Food52 

Crisp Topping:

- 2/3 cups almond meal (aka almond flour)
- 1 cup all-purpose flour
- 3/4 cup packed light brown sugar
- 1 teaspoon kosher salt
- 2 sticks butter, cold, cut into small pieces
- 1 1/3 cup rolled oats, quick-cooking oats, or a mix

Apple Mixture:

- 2 tablespoons freshly or bottled lemon juice
- 2 teaspoons cornstarch
- 2 heaping tablespoons brown sugar
- 3 teaspoons ground cinnamon
- 1/4 teaspoon kosher salt
- 1 teaspoon lemon zest (optional)
- 2 to 3 large apples—a juicy, firm variety, like Pink Lady, Honeycrisp, Jonagold, Braeburn, or a mix—peeled, cored, and cut into ~1/2-inch slices

- 2 large Granny Smith apples, peeled, cored and cut in to ~1/2-inch slices

1. Heat oven to 400°F.
2. In a large bowl, combine almond meal, flour, sugar, and salt, and whisk until integrated. Add the butter pieces, and, using your hands, squish the butter into the dry ingredients to create small chunks. The mixture should look like wet sand and hold together when pinched, with some unblended butter streaks visible throughout. 
3. Add the oats and roughly mix in. Set in refrigerator to chill while you prepare the apples.
4. Whisk the lemon juice with the cornstarch, brown sugar, cinnamon, salt, and lemon zest (if using) in a large bowl. Add the apple slices and toss gently until the slices are evenly coated. Transfer to a 8x8-inch baking dish (no need to grease it). You want about 1 inch of space between the apple slices and top of pan, so if you have too many apple slices, set them aside for a delicious snack.
5. Sprinkle the streusel evenly over the apples and transfer to oven. Bake for about 30 minutes, rotating the pan halfway through, until topping is crisp all over and browning in spots, and the apple mixture is bubbling up at the sides. The apple pieces should be tender and easily pierced with a fork. Let cool about 15 minutes before serving. I also enjoy this at room temperature with whipped cream.
6. Note: this recipe is very forgiving and can be easily 1.5x'd or doubled in a 9x13 pan. Keeps about 3 days in airtight container.