Preheat your oven to 350°F. Lightly grease an 8x12-inch casserole dish with butter and set aside.
Peel, core, and dice the apples into ½-inch chunks. Add the apples to the casserole dish, filling it about ¾ full.
Add maple syrup, flour, and salt to the top of the apples in the dish. Add cinnamon, allspice, and vanilla and stir well. Toss the apples in the sauce mixture until fully coated.
Place the dish in the oven and bake the apples for 20 minutes, allowing them to soften.
While the apples bake, make the oat topping. In a mixing bowl, combine flour, cinnamon, and salt. Cut in the cold butter slices using a pastry cutter until the mixture becomes crumbly. Stir in the oats and maple syrup.
Remove the casserole dish from the oven, and stir to re-coat the apples in the hot maple syrup mixture. Sprinkle the oat topping evenly over the apples. Bake for 30-40 minutes until the topping is golden brown and the apple filling bubbles around the edges.
Allow the crisp to cool for 15-20 minutes before serving. Allowing the crisp to cool gives the sauce a chance to thicken as it cools. Top with a scoop of vanilla ice cream or whipped cream for the ultimate fall dessert.