In a medium size bowl combine oats, flour, brown sugar, cinnamon, and baking powder. Add butter and cut in with a pastry blender or fork until well combined. Refrigerate while you prepare the apple filling.
(I use a Johnny Apple Peeler to peel, core and slice the apples all at once. It makes the process a lot faster!)
In a small bowl stir together melted butter and flour until smooth. Add lemon juice, milk and vanilla and stir. Stir in brown sugar, cinnamon, and nutmeg.
Pour butter mixture over apples and toss to coat. Pour apple mixture into an 8x8-inch baking dish and spread into an even layer. Sprinkle crumble topping evenly over the apples.
Bake for about 35 minutes or until golden brown and top is set. Remove from oven and allow to cool for at least 10 minutes before serving.
Serve with vanilla ice cream and homemade caramel sauce, if desired.
Make Ahead Instructions: Make the apple filling as instructed but add an extra splash of lemon juice to keep the apples from turning brown. Store in an air-tight container, in the fridge, for up to one day. Make the crumb topping and store separately in the fridge.Freezing Instructions: Prepare and bake apple crisp as directed. Cool completely, then cover with a double layer of aluminum foil. Freeze for up to 3 months. Thaw overnight in the refrigerator, then warm in a 350°F oven for 20-25 minutes or until heated through.Gluten Free Apple Crisp: Substitute Gluten-free flour and oats.Apple Crisp Without Oats: Omit the oats and add an extra 1/3 cup flour and 1/3 cup brown sugar.Crockpot Apple Crisp: Place prepared apple filling in the bottom of the slow cooker. Add crumble topping. Place a few paper towels on top of the oat topping, to keep condensation from the lid from making the topping soggy. Cook on high for 3-4 hours, or low for 6-8 hours until apples are tender.Instant Pot Apple CrispCaramel Pecan Apple CrispDutch Apple pie: Pour the apple filling into an unbaked pie shell. Add the crumb topping on top. Bake for about 45 minutes, until apples are tender. Cool for 2 hours before slicing and serving.