Remove white papery skin from garlic heads (do not peel or separate the cloves). Cut off top portions of garlic heads. Wrap garlic heads in foil. Bake at 350° 1 hour; cool 10 minutes. Squeeze garlic heads to extract pulp. Discard skins. Place garlic pulp, mustard, and oil in a food processor; process until smooth. Stir in chives and thyme leaves.
Increase oven temperature to 425°.
Trim fat from lamb. Cut 12 3/4-inch slits in lamb; place a fresh garlic slice in each slit. Sprinkle surface of lamb with 1/2 teaspoon salt and pepper; rub with roasted garlic paste mixture. Press breadcrumbs over surface of lamb. Place lamb on a broiler pan. Insert meat thermometer into thickest part of lamb, making sure not to touch bone.
Bake at 425° for 10 minutes. Decrease oven temperature to 325°. Bake an additional 2 hours and 10 minutes or until thermometer registers 140° (medium-rare) to 155° (medium). Remove lamb from rack; place on a shallow serving platter. Lightly cover with foil; let stand 15 minutes.
Drain fat from bottom of pan (do not scrape pan). Place broiler pan on stovetop over medium-high heat. Add broth, and bring to a boil, scraping to loosen browned bits.
Combine red wine and cornstarch, and stir with a whisk. Add to beef broth, and return to boiling. Cook 1 minute or until mixture is slightly thick, stirring constantly. Stir in 3/4 teaspoon salt, and serve immediately with lamb. Garnish with thyme sprigs, if desired.
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