• Preheat oven to 350° Fahrenheit. Line a 12-well muffin tin with muffin/cupcake liners. Set aside.

Make the Tart Crust

• Blend oats into a fine texture using a food processor or blender. Set aside 1/2 cup of the blended oat flour for later.
• Add brown sugar, cinnamon, and salt. Process again to combine. Add cubed butter and maple syrup. Pulse until the mixture is evenly combined and moist.
• Measure 2 tablespoons of crust mixture into each well of the lined muffin tin. Using the tablespoon, press the mixture down and up against the sides of the liner to form a compressed tart shell.
• Par-bake crusts for 7 minutes. Let cool while you prep the apple filling. (If the centers have puffed up, lightly push them back down.)

Make the Cinnamon Apple Filling

• Core, peel, and slice apples into thin slices. Cut the slices in half, so they're about 1½ inches long. Place into a microwave-safe bowl and sprinkle with 1 tablespoon of water. Microwave for 2 minutes.
• Let cool, then drain the water. Spread apple slices out on a layer of paper towels. Pat dry to remove any excess moisture.
• Place apple slices back in the bowl. Toss with cinnamon and nutmeg. Layer inside par-baked tart shells.

Make the Crumb Topping

• Combine the reserved oat flour (½ cup), brown sugar, and diced butter in a bowl. Use your hands to crumble the mixture together. Sprinkle on top of the apple tarts.
• Bake for 30-35 minutes, until the crumble topping turns golden brown. (Be sure to check them early in case they start to brown sooner. If they still aren't browning after 35 minutes, set the oven to broil and place them under the broiler for 30 seconds.)
• Let cool, then remove the cupcake liners and serve!