Coat slow-cooker bowl with nonstick cooking spray. Combine milk and water with cereal in prepared slow cooker bowl. Stir in 1/2 cup of the blueberries, the ginger and 3/4 tsp of the salt. Cover and slow cook on LOW for 9 hours.
In the morning, toast walnuts in a skillet over medium heat, 5 minutes. Stir remaining 1 1/2 cups blueberries, remaining 1/4 tsp salt and the honey into cereal mixture.
Spoon half of the cereal into bowls; top each serving with a thin slice of butter and some of the walnuts. Serve warm; save remainder for another day.