1. Melt 2 tablespoons butter in a large saute; pan over medium heat. Add onion and garlic; cook, stirring occasionally, 5 minutes. Add chard stems; cook, stirring occasionally, until stems begin to soften, about 7 minutes. Stir in half of the chard leaves and 2 tablespoons water; cook, stirring occasionally, until very tender, about 15 minutes. Transfer to a large bowl; cover. Add remaining chard leaves and 2 tablespoons water to pan; cook, stirring occasionally, until very tender, about 15 minutes. Stir into bowl.

2. Melt remaining 4 tablespoons butter in a small saucepan over medium-low heat. Add flour, stirring to avoid forming lumps. Gradually whisk in milk and nutmeg; season with salt and pepper. Raise heat to medium; bring to just below a boil, stirring occasionally; cook 2 minutes more, whisking constantly. Whisk in cream. Pour into chard mixture; stir to combine. Serve immediately.