PREP: Line a sheet pan with a silpat liner or parchment paper (just in case anything bubbles over) and lightly grease a 9 x 9 inch baking pan (or 2 quart size pan). Set aside.
TOPPING: In a large bowl, toss together the flour, oats, brown sugar, cinnamon, and salt. Stir until combined. Cube the butter and add it to the dry ingredients. Toss to coat the butter and use a pastry cutter to work it in. Finish by using your fingertips to work and press the butter into the dry ingredients until well dispersed and it resembles crumbs. Place in the fridge to chill for 15 minutes.
FILLING: Peel the apples and cut into 1/2 inch pieces. Add to a large bowl and toss with the remaining filling ingredients: the lemon juice, white sugar, cinnamon, cornstarch, and vanilla. Toss gently until well combined and apples are evenly coated.
ASSEMBLY: Pour the the apple filling (and any accumulated juices) to the prepared 9 x 9 inch pan. Spread apples into an even layer. Sprinkle the crisp topping evenly over the apples but do not press it down. Place in the fridge for 20 minutes. Then set the baking dish in the center of a large pan. Preheat the oven to 350 degrees F.
BAKE: Bake for 40 to 50 minutes (45 minutes is the sweet spot for me!), or until the crisp topping is golden brown, the apples are fork tender, and the filling is very slightly bubbling around the edges. (If the topping is browning too much, lightly cover the top with foil)
SERVE: Let the crumble cool on a rack for 15 to 20 minutes before serving. Serve warm and top with vanilla bean ice cream or fresh whipped cream