Layer half the potatoes, 1/4 cup onion, 1/8 teaspoon salt, 1/8 teaspoon nutmeg and 1 tablespoon butter in slow cooker. Repeat layers. Cover and cook on high 4 hours. Remove potatoes with slotted spoon to serving dish. Blend milk and flour in small bowl until smooth. Stir mixture into slow cooker. Add cheese; stir to combine. Cover and cook until slightly thickened, about 10 minutes. Stir. Pour cheese mixture over potatoes and serve. Garnish with chopped green onions, if desired.