Preheat the oven to 500°. Preheat a pizza stone or generously oil a large baking sheet. In a medium saucepan, melt the butter over moderate heat. Stir in the 3 tablespoons of flour until a paste forms. Gradually pour in the milk, whisking until smooth. Bring the béchamel sauce to a simmer over moderately high heat, whisking constantly, until thickened, about 4 minutes. Reduce the heat to low and cook, whisking often, until no floury taste remains, about 10 minutes. Remove from the heat and stir in the goat cheese until melted. Season the sauce with nutmeg, salt and pepper.