Grease the slow cooker insert well with butter or cooking spray. Combine the steel-cut oats, 4 cups water, 1 cup milk, maple syrup, salt, vanilla, and cinnamon stick. Stir, cover, and cook on low for 7 to 8 hours.

Carefully remove the lid, discard the cinnamon stick, and stir until creamy. If the oatmeal is too thick, stir in a little warm milk to loosen it.

Spoon into bowls and top with maple syrup, toasted nuts, fresh fruit, or whatever you like.