Preheat your oven to 425°F (220°C).

Sauté the onions and garlic in olive oil until translucent.

In a mixing bowl, combine sautéed onions, garlic, spinach, breadcrumbs, Parmesan, thyme, salt, and pepper.

Prepare the beef tenderloin by butterflying it along the length, leaving one side attached.

Stuff the tenderloin with the prepared mixture and tie it with kitchen twine.

Season the outside of the tenderloin generously with salt and pepper, and drizzle with olive oil.

Place the stuffed tenderloin in a roasting pan and pour beef broth around it.

Roast the tenderloin for about 40-50 minutes, or until the internal temperature reaches 130°F (54°C) for medium-rare.

Remove from the oven and let the beef rest for 15 minutes before slicing.

Slice and serve with your choice of sides.