Preheat oven to 400. Snap off and discard woody bases from asparagus. If desired, scrape off scales. In 15x10x1 baking pan, combine asparagus, sweet pepper and onion. Drizzle with oil; toss gently to coat. Spread in single layer. Sprinkle with salt and pepper.
Roast about 20 minutes or until asparagus is crisp-tender. Transfer to serving platter; sprinkle with Gruyere cheese. Let stand about 2 minutes or until cheese is melted.