BLUEBERRY FILLING: Prepare a 9x9-inch pan by lightly coating with nonstick spray. Toss blueberries, cornstarch, sugar (See Note 1), salt, vanilla extract, and lemon juice together in a large bowl. (See Note 2) Toss until combined and then place in the fridge while making the topping.
CRUMBLE TOPPING: In a large bowl, toss together the flour, oats, brown sugar, optional cinnamon and nutmeg, salt, and baking soda. 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.
ASSEMBLY: Pour the blueberry filling (and any accumulated juices) into the prepared 9x9-inch pan. Spread into an even layer. Sprinkle the topping evenly over the blueberries but do not press it down. Place the unbaked Blueberry Crisp in the fridge for 20 minutes. Preheat the oven to 350 degrees F
BAKE: Just before baking, set the baking dish in the center of a large pan (to catch any drips). Bake for 40 to 50 minutes (45 minutes is the sweet spot for me!), or until the crisp topping is golden brown, the blueberries are tender, and the filling is very slightly bubbling around the edges. (If the topping is browning too much, lightly cover the top with foil). Let the crisp cool on a rack for 15 to 20 minutes before serving. Serve warm with vanilla bean ice cream or fresh whipped cream.