In a skillet over medium heat, melt 2 tablespoons butter. Add onions, bay, thyme, salt and pepper, and cook to caramel in color and very soft, 25-30 minutes. Deglaze pan with sherry or brandy.
When onions are super soft and sweet -- but a few minutes shy of perfection -- begin your béchamel and preheat oven to 375°F. Organize bread slices on a small baking sheet. Melt remaining 2 tablespoons butter over medium heat in small pan. Whisk in flour, cook1 minute then whisk in milk. Thicken enough to coat the back of a spoon. Season with salt, pepper and nutmeg, and remove from heat. Stir in Dijon.