Place rack in the center of your oven and preheat oven to 400 degrees. Line a large rimmed baking sheet with tin foil, then set an ovenproof wire rack on top. Coat rack with nonstick spray. Set aside.
In a large bowl, stir together apple, red onion, garlic, cinnamon, salt, pepper, egg white, and panko. Add ground chicken and lightly mix with a fork until just combined, being careful not to compact the chicken.
Divide the mixture into quarters, then separate each quarter into 4 balls. You should now have a total of 16 meatballs.
Press a cube of cheddar into the center of each ball, then close the chicken around the cheese to seal it.
Arrange meatballs on the prepared baking sheet. Bake for 16 to 18 minutes, until cooked through.
To finish, place the cooked meatballs in the bottom of a slow cooker. In a small saucepan over medium-low heat, stir together the butter, apple butter, and barbecue sauce until the butter melts. Pour sauce over the top of the meatballs, toss gently to coat, and keep warm in the slow cooker.
Notes
STOVETOP DIRECTIONS: To finish the meatballs on the stovetop, stir together the apple butter and barbecue sauce in a large skillet. Add butter and stir until melted. Add cooked meatballs to the skillet, with a spoon, gently coat the meatballs in sauce. Place a lid on top of the skillet and reduce heat to low to keep warm.
MAKE-AHEAD: Unbaked meatballs can be prepped up to 1 day in advance. Place formed, unbaked meatballs in an air-tight container lined with plastic wrap, fit the container with a lid, and refrigerate until ready to bake.
TO STORE: Baked meatballs can be refrigerated in an air-tight container for up to two days.
TO FREEZE: Baked meatballs may be stored in the freezer for up to 3 months. Defrost overnight in the fridge before warming.
TO REHEAT: Reheat the meatballs gently in the microwave or on the stovetop.