Mix the fennel, coriander, cumin, celery, and caraway seeds, 2 tsp. salt, and 1 tsp. pepper on a large cutting board; roll over them with a heavy rolling pin until lightly cracked. Roll the roast in the seeds to coat on all sides. Gather any remaining seeds and pat them onto the beef. Set the beef aside at room temperature for 1 hour.
Meanwhile, position a rack in the center of the oven and heat the oven to 200°F.
Position the beef bone side down in a large, heavy-duty, flameproof roasting pan. Roast until a probe or instant-read thermometer inserted into the center of the eye registers 120°F to 125°F for rare, 130°F to 135°F for medium rare, or 145°F for medium, between 4 and 5 hours. Begin checking the temperature after 4 hours to avoid overcooking. Let the beef rest in the roasting pan, uncovered, at room temperature for 1 to 1-1/2 hours.
Raise the oven temperature to 500°F. Return the beef to the oven and roast until the seeds begin to brown, about 12 minutes.
Transfer the beef to a carving board. Pour off any fat from the roasting pan (reserve, if desired, to make Yorkshire Pudding). Set the roasting pan over medium-high heat, add the beer, and bring to a boil, stirring down the foam and scraping up any browned bits from the pan with a wooden spatula. Boil until the liquid is reduced to 3/4 cup, 10 to 15 minutes. Whisk in the broth and mustard and continue boiling, whisking often, until reduced to about 1-1/2 cups, about 10 minutes. Whisk in the butter just until emulsified, remove from the heat, and season to taste with salt and pepper; transfer to a gravy boat for serving.
To carve, remove the meat from the bones by running a long, thin carving knife between the eye of meat and the bones, along the interior curve of the bones. Then cut the beef into thick slices and lightly sprinkle with sea salt. Serve the beef with the sauce.