Cook rotini to desired doneness as directed on package. Drain; cover to keep warm.
Meanwhile, heat oil in 12-inch nonstick skillet over medium-high heat until hot. Add onion and garlic; cook and stir 1 to 2 minutes or just until garlic begins to brown.
Add sweet potato and bell pepper; cook and stir 2 minutes. Add water; cover and cook 2 to 3 minutes or until vegetables are tender.
Add cooked rotini and spinach; stir gently to mix. Cook uncovered until thoroughly heated. Remove from heat; stir in pesto until well mixed. If desired, add salt and pepper to taste. Sprinkle with cheese.