In a large nonstick skillet, heat oil over medium-high heat. Add sausage, onion and celery; cook and stir until sausage is browned, 6-8 minutes. Add garlic; cook 1 minute longer.
Stir in broth, water and pepper flakes; bring to a boil. Stir in couscous. Reduce heat; simmer, covered, until liquid is absorbed, 10-12 minutes. Remove from heat; let stand, covered, 5 minutes. Stir in parsley. If desired, sprinkle with cheese.
