In bowl, mix first 4 ingredients; season. Form into 20 meatballs. In large skillet, heat oil over medium. Add meatballs; cook, turning often, until browned all over, 8 minutes. Drain fat. Add tomato sauce, stock, couscous and cumin to meatballs. Simmer over low, stirring often, until couscous is tender, about 8 minutes; season. Top with yogurt and mint.