Heat the oven to 350ºF. Grease a 12-cup muffin tin and set aside.
In a small bowl, beat together the egg, banana, milk, sweetener, and vinegar.
Add the dry ingredients directly into the bowl with the wet and stir to thoroughly combine. Fold in the apple.
Quickly make the topping my mixing it all together in a bowl.
Fill each cup 3/4 of the way full and top with about 1 tablespoon or so of crumble topping.
Bake on the center rack for 22 - 24 minutes until a cake tester inserted into the center comes out clean.
Cool in the pan for 2 - 3 minutes, then transfer to a wire rack and cool completely. Resist the urge to not break into them; cooling them will allow everything to set properly.
If desired, reheat in the toaster oven or in the microwave.
Notes
Storage Tips:
Room temperature: muffins can be stored in a sealed container at room temp for 2 days. Enjoy at room temp or heat in the microwave for 10 seconds.
Refrigerator: muffins can be stored in a sealed container in the fridge for up to 4 days. Heat in the microwave for 10 seconds before eating.
Freezer: wrap the muffins in foil, then store in a sealed container for up to 3 months. Reheat in the microwave for 20 seconds.