• Preheat the oven to 375°F. Lightly coat a baking dish just large enough to hold the tenderloins with nonstick spray.

Prepare the turkey for stuffing:

• Place the turkey tenderloins flat on a cutting board. Cut a lengthwise slit down the center of each, stopping about 1/2 inch from both ends and being careful not to cut all the way through.
• Gently open each slit to form a pocket. Season the inside and outside of the tenderloins with salt and pepper, then place them pocket-side up in the prepared baking dish.

Make the stuffing:

• Melt 2 tablespoons of butter in a skillet over medium heat. Add the onion and celery, season with a pinch of salt and pepper, and sauté until softened. Remove from the heat and let cool for at least 5 minutes.
• Place the bread cubes in a colander and moisten them evenly with water. Squeeze gently with your hands until the bread is damp but not dripping, then transfer to a mixing bowl.
• Add the salt, a few grinds of black pepper, beaten egg, sage, thyme, and parsley to the bread. Add the cooled onion-celery mixture (scraping in any butter from the pan) and mix until well combined.
• Spoon the stuffing into the pockets, pressing gently so it fills evenly without packing it too tightly, and mound slightly on top. If you have extra stuffing, spoon it into the baking dish between the tenderloins or bake it separately in a small oven-safe dish.

Bake the turkey tenderloins:

• Spray one side of a piece of foil with nonstick spray (to prevent sticking) and cover the baking dish loosely, sprayed-side down. Bake for 30 minutes.
• Remove the foil and continue baking until the stuffing is lightly browned and an instant-read thermometer inserted into the thickest part of the tenderloins registers 160°F.
• Remove from the oven and let the tenderloins rest for 5 minutes before slicing (the temperature will continue to rise slightly as they rest).

Make the gravy:

• While the turkey bakes, melt 3 tablespoons of butter in a medium saucepan over medium-high heat. Add the mushrooms and cook until their liquid evaporates and they begin to brown. Season with salt and pepper.
• Add 2 cups of the chicken broth and bring to a simmer.
• In a small bowl, whisk the flour with the remaining 1/4 cup broth until smooth. Add the mixture gradually to the simmering mushrooms, stirring constantly until blended.
• Simmer until the gravy thickens enough to coat the back of a spoon. Turn off the heat, cover, and keep warm until ready to serve.