2 1/2 cups fresh breadcrumbs made from crustless french bread
For sauce: Boil all ingredients in large saucepan until reduced to 2 1/3 cups, about 20 minutes. Cool. Puree in blender. DO AHEAD: Can be made 5 days ahead. Cover and refrigerate.
For lamb: Pulse grapes, olives, chopped rosemary, vinegar, and garlic clove in mini processor until olives are chopped. Transfer to bowl. Mix in ground lamb,1/2 teaspoon salt, and 1/4 teaspoon pepper.
Sprinkle racks of lamb with salt and pepper. Heat 1 tablespoon oil in large skillet over medium-high heat. Add 1 rack, meat side down, and cook until browned, about 5 minutes. Set on rimmed baking sheet, meat side up. Repeat with 1 tablespoon oil and remaining rack; reserve skillet. Cool lamb completely.
Add 1/2 cup broth to skillet. Boil until reduced to glaze, scraping up browned bits. Add to sauce. Cover and chill.
Spread each rack with 1 tablespoon mustard. Press half of lamb sausage over top of each rack (layer will be thin). DO AHEAD: Can be made 1 day ahead. Cover and chill.
Preheat oven to 425°F. Combine breadcrumbs, 2 tablespoons oil, and 2 tablespoons mustard in medium skillet. Toss over medium heat until beginning to color, about 5 minutes. Press crumbs over sausage on each rack. Cut through crumb crust (not lamb) between bones to score. Roast until thermometer inserted into center registers 135°F, about 30 minutes for medium-rare.
Transfer lamb to platter; let rest 10 minutes. Pour juices from baking sheet into sauce. Simmer in small saucepan until reduced to about 1 1/3 cups, about 9 minutes (sauce will thicken slightly). Season with salt and pepper.
Cut lamb between bones into chops. Set two on each plate. Spoon sauce over.
WHAT TO DRINK: Martín Códax 2005 Ergo Tempranillo (Spain, $14), with lively fruit,full body, and supple texture.