Heat charcoal or gas grill. Brush onion, bell peppers, asparagus, mushrooms and tomatoes with 1 tablespoon olive oil. Sprinkle with salt and pepper. Place onion, bell peppers and asparagus on grill. Cover grill; cook over medium heat 9 to 12 minutes, turning occasionally, until vegetables are crisp-tender. Add mushrooms during last 6 minutes and tomatoes during last 4 minutes of grilling. Cut grilled vegetables into bite-size pieces. Set aside.