Remove white papery skin from garlic heads (do not peel or separate the cloves).
Place garlic heads in a 1-quart baking dish; add 1/2 cup broth. Cover and bake at 350 for 1 hour. Cool 10 minutes; remove garlic heads, reserving cooking liquid. Separate cloves; squeeze to extract garlic pulp, and discard skins.
Place reserved cooking liquid, garlic pulp, 1 1/2 cups broth, 1 teaspoon marjoram, and 1/2 teaspoon salt in a food processor; process until smooth. Set garlic sauce aside.
Combine 3 teaspoons marjoram, 1/2 teaspoon salt, 8 garlic cloves, and shallots in a food processor; process to a coarse paste. Trim fat from roast; make 3/4-inch-deep slits into roast. Spoon 1/4 teaspoon marjoram paste into each slit; rub roast with remaining paste.
Sprinkle roast with pepper. Cover and chill 1 hour.
Preheat oven to 32
Place roast on a broiler pan; insert meat thermometer into thickest portion of roast.
Bake at 325 for 2 hours or until thermometer registers 145 (medium-rare) or desired degree of doneness.
Place roast on a platter; cover with foil.
Let stand 15 minutes. Set pan and drippings aside.
Add brandy and 1/2 cup garlic sauce to broiler pan, scraping the pan to loosen browned bits.
Combine brandy mixture and remaining garlic sauce in a saucepan; bring to a boil.
Cut roast across grain into very thin slices; serve with garlic sauce.