Melt the 1 stick unsalted butter in a saucepan over medium-low heat. Whisking constantly, slowly add the 6 tablespoons all-purpose flour, continuing to whisk until all the butter is absorbed and the mixture creates a very thick paste.
Still whisking constantly, add the 3 to 3 1/2 cups milk very, very slowly and whisk constantly until you end up with a paste that's sorta thick and spectacularly smooth. (When we make this, we slowly add 3 cups milk for starters and then if the sauce seems too thick, we add up to 1/2 cup more milk, a little at a time.) Do not stop whisking at any point and do not allow the sauce to boil.
When the sauce is smooth, whisk in the 6 tablespoons grated Parmesan cheese.
Remove the pan from the heat and whisk in the 1 large egg until completely incorporated. Using a spoon or a spatula, gently fold in the 2 tablespoons whipped cream, if desired. Season with sea salt and freshly ground black pepper to taste. (You may not need any salt since the Parmesan sorta takes care of the saltiness.)
Preheat the broiler. Place the 8 slices toast on a rimmed baking sheet lined with foil or parchment.
Top each slice with a liberal amount of leftover roast turkey and a few slices of 4 Roma tomatoes and smother it with a generous amount of sauce. (Note that any exposed portion of toast will crisp and brown in the oven, so bear this in mind when you slather the sauce over the toast and turkey with sauce. If you have some sauce left over, no big deal, just refrigerate it and make another hot brown tomorrow.)
Sprinkle with additional Parmesan cheese.
Slide the baking sheet under the broiler until the sauce is speckled brown and bubbly. Watch it carefully as the sandwiches will go from almost perfect to past perfect in seconds.
Criss-cross 2 strips of bacon on top of each open-face hot brown sandwich. Serve the sandwiches immediately. (As if you could wait!)