Here's What You'll Need
pork tenderloin, salt, black pepper, fontina, roasted red pepper, baby spinach, red onion, shiitake mushrooms, rosemary, lemon
Trim the pork of any visible silverskin. Slide the tip of a knife underneath the silverskin tilted up, then slide the knife while pulling the membrane up. Discard.
Place on a cutting board, then hold the knife parallel to the length of the pork. Make a shallow cut from top to bottom, about 1" thick. Repeat 1-2 more times, being careful not to cut through the tenderloin. Season inside with salt.
Place oil and butter in a skillet. Add onion and mushrooms. Cook until mushrooms begin to brown. Add roasted red pepper, spinach, rosemary, lemon zest, and salt, cook until spinach has wilted. Remove from the heat.
Add a thin layer of fontina on the pork tenderloin, leaving the outer ½" clear. Spoon the mushroom and spinach mixture down the center of the butterflied pork.
Roll the tenderloin into a cylinder, try to enclose the filling. Use kitchen twine to secure together. Season the outside of the pork on all sides with salt and pepper.
Place pork over indirect heat, turning every 5 minutes until an internal temperature of 145F has been reached, about 25 minutes. Allow meat to rest for 10 minutes before slicing and serving.