Preheat oven to 375 degrees F.
In a large saucepan, combine beef broth and port. Bring to a boil. Cook over high heat until the volume is reduced by about one-third, about 30 minutes. Whisk in tomato paste. Remove pan from heat and set aside. This can be a done a day in advance or earlier in the day if you'd like.
Trim the tops off the heads of garlic with a sharp knife to expose the tops of the cloves and remove some of the papery outer skins. Drizzle them with olive oil and season with a little sea salt.
Pat the beef dry with paper towels. Fold the thinner, tapered end underneath to create an evenly sized roast and then use kitchen twine to tie it at 2-inch to 3-inch intervals, or as needed. Sprinkle both sides of the beef with salt, thyme, and pepper. Add olive oil to a large roasting pan and set it over MEDIUM-HIGH heat (you may need to use two burners). Add the tenderloin to the hot pan and sear for about 4 minutes per side. Place trimmed garlic around the beef and transfer the pan to preheated oven. Roast the beef until meat thermometer inserted into the center of the thickest portion registers 125 degrees F (rare), 130 degrees F (medium rare), or 135 F (medium), about 40 minutes. Watch closely and remove from oven as soon as the beef reaches desired temperature. Immediately transfer the beef to cutting board and tent loosely with foil to rest for at least 15 minutes.
Transfer the heads of garlic to a small baking dish, drizzling with additional olive oil, if necessary. Increase the oven temperature to 400 degrees F and return the garlic to the oven for an additional 5 to 10 minutes, or until very tender and golden brown.While the beef rests, finish the gravy.
Spoon fat deposits off top of pan drippings in the roasting pan. Place pan over MEDIUM-HIGH heat. Add broth mixture and bring to a boil, stirring to scrape up any browned bits. Mix 3 tablespoons softened butter and flour in small bowl to form smooth paste and whisk it into broth mixture. Simmer until gravy thickens. Whisk in remaining tablespoon of butter. Taste and season with salt and pepper, only if needed. Transfer to a gravy boat or small bowl.
Slice the beef tenderloin into medallions and serve with the roasted garlic and Port Wine Gravy