First, preheat the oven to 350ºF and line a muffin tin with paper muffin cups and then spray the paper cups with non-stick cooking spray.
Add the apple cubes, orange juice, orange zest, cinnamon, nutmeg, and salt to a bowl and toss until the apples are coated in the juice and spices. Set aside.
In a separate large bowl, mix together crumble topping. Place rolled oats, flour, brown sugar, cinnamon, and salt. Then, add in a stick of softened butter and use a fork (or your hands) to cut the butter into the dry ingredients until it forms little crumbles.
Add 2 tablespoons of the crumble mixture to each muffin cup and use the back of a tablespoon to press the crumble into the bottom of the muffin cup to create a well with the crumble. Once each muffin tin has a crumble well, place the muffin tin into the oven and bake for 10 minutes.
Remove the muffin tin from the oven and let the crumble crust cool for 10 minutes.
Next, scoop 1.5-2 tablespoons of the diced apples into each crust well and then cover the diced apples with 1-2 tablespoons of the remaining crumble. Try to pour the crumble over the top and sides of the apples gently pressing the crumble into the apples and connecting the crumble with the bottom crust.
Place the muffin tin back into the oven and bake for 20-25 minutes or until the top of each mini apple crisp is golden brown.
Remove the muffin tin from the oven and let the apple crisps rest for 10-15 minutes.
Serve warm with ice cream or whipped cream or enjoy cold throughout the week.