Preheat oven to 425°.
Sprinkle lamb with 1/4 teaspoon salt and 1/4 teaspoon pepper. Combine mustard, rosemary, and thyme. Rub lamb with half of mustard mixture. Combine 1/4 teaspoon salt, 1/4 teaspoon pepper, remaining mustard mixture, wine, and oil in a large bowl. Add mushrooms; toss well.
Place lamb on the rack of a broiler pan or roasting pan coated with cooking spray. Spoon mushroom mixture into bottom of pan coated with cooking spray; place rack in pan over mushrooms. Insert a meat thermometer into the thickest part of 1 chop.
Bake at 425° for 25 minutes or until thermometer registers 140°, stirring mushroom mixture after 12 minutes. Place 1 lamb chop on each of 2 plates; keep warm. Remove rack from pan; add broth to mushroom mixture in pan. Place pan over medium-high heat; cook 5 minutes or until liquid almost evaporates. Serve lamb with mushroom mixture and couscous.