To make the gravy, heat a small saucepan over medium-high heat and melt the butter. Whisk in the flour to blend. With a wooden spoon, stir constantly until the mixture is the color of peanut butter, about 7 minutes. This is called making a roux.
Add the onion and celery and saute until soft, about 2 minutes.
Whisk in the chicken broth, Baby Bam, and salt. Bring to a boil, reduce the heat to medium-low, and simmer until the mixture thickens and the floury taste is gone, about 20 minutes.
To assemble the sandwiches, spread about 1 1/2 teaspoons of the mayonnaise on each slice of bread. Arrange equal amounts of the turkey on top of each slice of bread. Ladle equal amounts of the gravy (almost 1/2 cup each) over each open-faced sandwich. Serve immediately, topped with sauteed mushrooms if you like.
YIELD: 4 open-faced sandwiches and 1 3/4 cups gravy
In a medium skillet over high heat, melt the butter. Add the mushrooms, salt, and pepper, and saute, stirring occasionally, until mushrooms are soft, have released their liquid, and are golden, about 3 minutes.
Divide the Sauteed mushrooms evenly over the gravy among the tops of the sandwiches and serve.