1. Cut the top half inch or so off the top of the apples. Use a spoon or melon baller to scoop out the core of each apple, leaving the bottom intact.In a medium bowl, stir together the oats, brown sugar, cinnamon, ginger, cloves, and walnuts until well mixed.
2. Cut the butter into chunks, then work it into the oat mixture until crumbly.Stuff the oat and butter mixture into the cavity of each apple. Nestle the apples into the slow cooker in one layer, place the lid on top, and cook on high for 1.5 to 2 hours, or until the apples are soft.Once the apples are soft, carefully lift them from the slow cooker and serve immediately (as is, topped with ice cream, or whipped cream).