Whisk together chicken broth and cornstarch till well blended; set aside.
Prepare lentil couscous according to package directions; place on a serving platter and keep warm.
In a large skillet over medium-high heat, sauté lamb in the oil until nicely browned all over, about 4 to 5 minutes; remove from skillet and keep warm.
Add onions and garlic to the skillet and sauté till onions are translucent.
Return lamb to skillet then add vinegar and mustard. Cook and stir for 3 to 4 minutes to blend ingredients and allow lamb to cook through.
Add reserved broth/cornstarch mixture and bring to a low boil, stirring constantly till thickened; season with salt and pepper to taste.
Pour lamb cubes and sauce over lentil couscous and serve.