Bring 3/4 cup water to boil in a medium saucepan; gradually stir in couscous. Remove from heat; cover and let stand 5 minutes. Fluff with a fork.
While couscous stands, place sweet potato on a microwave-safe plate. Microwave at HIGH 5 minutes or until tender.
Combine vinaigrette, pepper, and salt in a large bowl; stir well with a whisk. Add couscous, sweet potato, soybeans, and spinach; toss gently to coat. Top each serving with cheese; sprinkle evenly with onions.