Preheat the oven to 350°F. Butter a 1.5 L baking dish and set it on a large rimmed baking sheet. Set aside.
Oat crumble topping
Mix the butter, brown sugar, all-purpose flour, oats, and salt. You can do this with your fingertips, working the mixture until a cookie dough forms. Set aside.
Apple filling
Peel and core the apples then slice them.
Toss the apples with the sugar, lemon juice, flour (if using), vanilla bean paste, cinnamon, and nutmeg. Make sure the apples are evenly coated. Pour them evenly into the bottom of the prepared baking dish.
Scatter globs of the crumble topping over the apples, then sprinkle with the remaining 1 tablespoon of oats and 1 tablespoon of turbinado.
Bake the apple crisp for about 1 hour, until the apples are cooked through, the topping is golden browned, and the filling is bubbling.
Let cool on a wire rack. Serve warm or at room temperature.