In a 2-qt. saucepan, melt the butter over medium-low heat. Add the flour and cook, whisking constantly for 2 to 3 minutes. Do not let the mixture brown. Slowly whisk in the hot milk and bring just to a simmer, whisking frequently. Reduce the heat to low and cook, whisking often, until the sauce has thickened to a creamy, gravy-like consistency and no longer tastes of raw flour, 6 to 8 minutes for a single batch, 10 to 12 minutes for a double batch. Remove from the heat and whisk in the salt, pepper, and nutmeg. If not using right away, transfer to a bowl and press a piece of plastic wrap directly on the surface of the sauce to keep a skin from forming. Plan to use the sauce within 30 minutes because it thickens if it’s left to sit for too long. If that should happen, add a little warm milk and whisk well to thin it.