Combine the brown sugar, oats, flour, cinnamon, nutmeg and cloves in a medium size bowl.
Add the diced butter and gently combine by squishing the butter between your fingers. Small chunks of butter are fine. Set aside.
Core the apples using an apple corer or pairing knife. Make sure you don't cut all the way to the bottom, so the filling doesn't leak out. The bigger the hole the more filling you can stuff inside.
Add 1/2 cup of water to the bottom of the slow cooker, followed by the apples.
Stuff each apple to the top with the filling.
Let cook for 1.5 - 2 hours on high or 4 hours on low.
Half way through, add more filling to each apple if desired.
Once the apples are soft to the touch they're ready to serve.
Serve by itself or with a scoop of vanilla ice cream.