Toast the garlic, unpeeled, in a small skillet for 7-8 minutes over medium-high heat until they become spotty brown. Poke slits with a paring knife on all sides of the roast. Peel garlic and cut each clove into 3-4 slivers, then poke a sliver into each slit in the roast.
Combine mashed garlic, thyme and salt in a small bowl. Rub into all surfaces of the roast. Transfer roast to a plate and refrigerate, uncovered, at least 4 hours.
Combine Garlic Paste ingredients in small saucepan. Place over medium-high heat until it begins to bubble, then reduce to low and cook until garlic softens, about 30 minutes. Remove from heat and allow to cool completely. Strain oil into a small bowl. Remove thyme and bay and discard. Mash garlic cloves along with 1 tbsp of garlic oil, using a fork. Cover and refrigerate until roast is done resting.
Preheat oven to 450°F Place an empty, dark-colored, non-stick roasting pan in the oven to heat as well.
Remove roast from refrigerator and remove the rub with paper towels. Rub 2 tbsp garlic oil over surface of roast, then sprinkle with pepper. Place roast in roasting pan, fat side down. Roast 10-15 minutes, turning to brown all sides.
Lower oven to 300°F Turn roast fat-side up, and smear garlic paste over the top. Roast 50-70 minutes, until internal temperature is 120°F.
Remove roast to a carving board, tent with foil, and let rest 20 minutes.
Meanwhile, place roasting pan over a burner set to medium heat. Add chicken broth and beef broth. Simmer down until reduced to 2 cups. Drain any accumulated juices from carving board into jus. Pour through a sieve before serving.