Combine chicken broth and cornstarch, stirring well. Set aside.
Heat olive oil in a large nonstick skillet over high heat. Add lamb; sauté 4 minutes or until browned.
Prepare couscous according to package directions, omitting salt and fat.
Remove lamb from pan. Add onion and garlic; sauté 2 minutes or until onion is tender. Return lamb to pan. Stir in balsamic vinegar and mustard; cook 3 minutes or until liquid is reduced by half. Stir in chicken broth mixture; cook 2 minutes or until thickened. Stir in pepper.
Spoon couscous onto a platter; spoon lamb mixture over couscous.