Combine the sugar, oats, flour, baking powder, allspice, nutmeg and kosher salt in a large bowl. Add the butter and work into the dry ingredients until a crumbly texture is formed. Fold in the peach slices.
Butter the bottom and sides of a 3-quart cooker. Add the mixture to the slow cooker and cook on low for 3 to 3 1/2 hours. Serve immediately.