Pat the tenderloin completely dry with paper towels. Let sit at room temp for 1 hour. Drizzle with olive oil, then coat evenly with kosher salt and pepper.

Preheat oven to 425°F (220°C). If making the herb butter, mix the softened butter, minced garlic, chopped rosemary, lemon zest, and salt in a small bowl. Set aside. Lightly smash the peeled garlic cloves from the head.

Heat a large oven-safe skillet over high heat until very hot. Sear the tenderloin on all sides until a deep brown crust forms, about 3-4 minutes per side (10-12 mins total).

Remove skillet from heat. Tuck smashed garlic cloves and whole rosemary sprigs around the beef. If using, spread the herb butter over the top of the seared tenderloin.

Transfer the skillet to the preheated oven. Roast until the internal temperature reaches 125-130°F (for medium-rare, temp will rise during rest), about 15-20 minutes. Use a meat thermometer for accuracy.

Transfer beef to a cutting board, tent loosely with foil, and rest for 15-20 minutes. Do not slice yet.

(Optional) For sauce: Place skillet with drippings over medium heat. Add red wine, scrape up browned bits, and reduce by half. Add beef broth, simmer 5-7 mins. Off heat, swirl in cold butter until glossy. Strain if desired.

Slice the rested tenderloin into ½-¾ inch thick medallions. Arrange on a platter, spoon over a little sauce if using, garnish with fresh rosemary, and serve immediately.