Remove the outer leaves from the cauliflower and coarsely chop; separate the cauliflower into florets. Place the cauliflower and chopped leaves in the slow cooker with about an inch of water.
In a saucepan, melt the butter and whisk in the flour. Cook for two to three minutes, stirring constantly. Add the milk, whisking continuously to ensure that no lumps form. Stir in the grated cheese and the freshly grated nutmeg.