Preheat the oven to 400 degrees F and a cast-iron griddle to medium-high heat.
For the Mornay sauce: In a medium skillet over medium-low heat, add the butter and sweat the onions until softened, 3 to 5 minutes. Add the flour and toast for an additional minute without adding color. Bring the skillet to medium-high heat. Add the chicken stock and bring to a boil. Stir to incorporate and simmer for 3 minutes. Add the milk and reduce the heat to medium-low. Simmer until the starch has cooked out and the sauce has thickened, an additional 3 minutes. Add the nutmeg to taste. Reduce the heat to low. Stir in the Cheddar and Parmesan. Remove from the heat and keep warm. Season with salt and pepper.
For the sandwich: Melt the butter and mix with the garlic. Brush one side of the slices of the sourdough with the garlic butter. Griddle the toast on both sides until golden brown and toasted. Remove from the heat. Transfer to a baking sheet, garlic-butter-side up, and season with the flake salt.
Top the toast slices with about 1 1/2 ounces of turkey and 2 slices of bacon. Shingle the tomato slices over the bacon and turkey. Spoon approximately 1 ounce of Mornay sauce over the top and sprinkle with Parmesan. Place in the oven and toast until the Parmesan bubbles, about 5 minutes (finish under the broiler if desired). Remove and garnish with the scallions. Serve open face, 2 per serving.