Pat dry and lay the roast flat on a cutting board. Carefully make a lengthwise cut about two thirds of the way through to open it like a book. Pound to an even thickness of about 3/4 inch if necessary, then season the inside with salt and pepper.

In a skillet, heat 1 tablespoon of olive oil over medium heat. Add the onion and mushrooms, cooking until softened, about 5 to 6 minutes.

Add garlic and cook for an additional 30 seconds. Stir in the spinach until wilted.

Remove from heat and mix in Dijon, balsamic vinegar, Italian seasoning, breadcrumbs, Parmesan, and sun dried tomatoes. Taste and season with salt and pepper, then let cool slightly.

Spread the stuffing evenly over the opened tenderloin, leaving a small border. Roll it up tightly from the long side and tie with kitchen twine every 2 inches. Season the outside with salt and remaining pepper.

In an oven-safe skillet, heat the remaining tablespoon of olive oil over medium high. Sear the tenderloin on all sides until browned, about 1 to 2 minutes per side.

Transfer the skillet to a preheated 425°F oven and roast until the thickest part of the roast reaches 125°F for medium rare, about 20 to 25 minutes.

Move the roast to a cutting board and rest for 10 to 15 minutes. Remove twine and if desired, melt a tablespoon of butter over the top, sprinkle with parsley, then slice into 1 inch rounds.