Preheat the oven to 400°F (200°C).

Prepare the stuffing: In a skillet, heat olive oil over medium heat. Sauté garlic and mushrooms until soft, about 5 minutes. Add spinach and cook until wilted. Stir in cream cheese, breadcrumbs, mustard, and rosemary; season to taste.

Butterfly the beef tenderloin by slicing it lengthwise into the center, being careful not to cut all the way through. Open it like a book.

Spread the stuffing mixture evenly inside the opened tenderloin, pressing gently to compact it. Roll the beef tightly, starting from the non-cut side.

Secure the rolled beef with cooking twine in several places, then season the outside with salt and pepper.

In a large skillet, heat olive oil over medium-high heat. Sear the stuffed beef tenderloin on all sides until nicely browned, about 2-3 minutes per side.

Transfer the seared tenderloin to a roasting pan and cook in the preheated oven for 20-25 minutes for medium-rare, or until desired doneness is achieved.

Remove from the oven and let the tenderloin rest for 10 minutes before slicing.

Slice the beef tenderloin into medallions and serve with your choice of sides.