Prepare the beef tenderloin by trimming any excess fat and pouncing.

Create the stuffing by sautéing mushrooms, spinach, garlic, and thyme in olive oil.

Mix sautéed ingredients with breadcrumbs, Parmesan, salt, and pepper in a bowl.

Stuff the beef tenderloin with the prepared mixture and tie it with cooking twine.

Sear the stuffed tenderloin in a hot skillet until browned on all sides.

Transfer the tenderloin to an oven-safe dish and roast in the oven at 375°F (190°C) until cooked to your liking.

Let the tenderloin rest for 10-15 minutes before slicing.

Slice the tenderloin and serve it with your preferred sides.