1 standing beef rib roast, ribs frenched, fat trimmed to 1/4 inch, and tied at 1-inch intervals (5 1/2 to 6 pounds)
1 1/4 teaspoons fennel seed
2 teaspoons fresh thyme leaves
1 tablespoon plus 1 teaspoon coarsely chopped fresh rosemary leaves
Coarse salt and ground pepper
2 heads garlic
Let beef sit at room temperature 1 hour. Preheat oven to 275 degrees, with rack in lower third. In a spice grinder, combine fennel seed, thyme, and rosemary and process into a powder. Pat beef dry with paper towels and season with salt and pepper. Place a large broilerproof roasting pan across two burners and heat over medium-high. Add beef, fat side down, and brown on all sides, about 10 minutes total. Transfer beef to a rimmed baking sheet. Tilt pan and use paper towels to wipe any burned bits from bottom but leave drippings. When beef is cool enough to handle, sprinkle all over with herb mixture.
With a serrated knife, cut garlic heads in half crosswise. Arrange garlic halves in middle of pan; prop beef on top. Roast until garlic is tender and beef is medium-rare (an instant-read thermometer inserted in thickest part of roast should read 125 degrees), 1 3/4 to 2 hours. Transfer roast and garlic to a cutting board and tent loosely with foil; let rest 30 minutes.
To carve, separate the meat from the bones by sliding a knife between the ribs and the meat and cutting downward, following curve of bones. Then cut the ribs apart.
To serve, slice meat against the grain. Arrange on a serving platter with ribs and garlic.