Preheat oven to 450F. Line a baking sheet with foil and spray with cooking spray or olive oil. Toss together sweet potato, red onion, and olive oil on top of sheet. Roast for 20-25 minutes, until potatoes are tender and brown in spots.
Transfer to a large bowl and let cool completely. Toss with tomato, avocado, cilantro, and lime juice. Season with salt and toss to combine.