Preheat oven to 425 degrees F; position the oven rack in the center of the oven.
In a large skillet, cook pancetta slices, over medium heat, in batches, for 3 minutes, remove pancetta slices to a plate, and set aside.
Heat 2 tablespoons oil in same skillet on medium.
Season tenderloins with salt and pepper, then brown the outside, all over, in skillet about 6 to 8 minutes; remove to work surface to cool.
Meanwhile, in the same skillet, cook chopped pancetta in remaining tablespoon of oil, stirring, until golden, about 5 minutes.
Add butter and transfer chopped pancetta to a medium bowl.
Add 1/4 cup water to skillet and continue to cook while scraping up any browned bits; add liquid to the bowl.
To the bowl, add bread crumbs, parsley, thyme, 1/2 teaspoon salt and 1/2 teaspoon pepper; reserve skillet.
Slice tenderloins, lengthwise, 3/4 of the way through, then open like a book, cut side up.
Beginning in the center, slice each side on the diagonal 3/4 of the way through and open again like a book. You are trying to get a flat piece of meat that you can roll up.
Spread half of the stuffing mixture in center, leaving a 1 inch border, roll tenderloins up over the stuffing mixture, arrange half the pancetta slices over the top in an overlapping pattern, and tie with kitchen string at 2 inch intervals; repeat with other tenderloin.
Roast stuffed tenderloins until they register 120 degrees F with meat thermometer, about 25 minutes.
Transfer to a cutting board and tent with foil.
Pour the pan juices into the reserved skillet, add mushrooms, season with salt and pepper, and cook over medium high heat until browned, about 10 minutes.
Stir in the port wine and cook an additional minute.
To serve, slice roasts and spoon mushrooms and sauce on top.