Preheat oven to 400° Fahrenheit. Add cut sweet potatoes to large bowl and pour avocado oil over. Toss to fully coat sweet potatoes in oil, then spread sweet potatoes out on baking sheet in one even layer.
In small bowl, whisk together basil, oregano, onion powder, garlic powder, dried thyme, and salt. Sprinkle spice mixture over sweet potatoes in small increments until sweet potatoes are evenly, liberally coated with spices. Note: you may not use entire spice mixture, but sweet potatoes should be very well-coated.
Place baking sheet in preheated oven. Roast sweet potatoes 25 minutes, then remove baking sheet from oven. Use spatula to carefully flip sweet potatoes over, then return baking sheet to oven. Bake another 10 to 20 minutes or until potatoes are golden-brown and beginning to crisp. Be careful not to burn potatoes.
Remove baking sheet from oven. Transfer sweet potatoes to large bowl and set aside for 3 to 5 minutes or until potatoes have cooled slightly but are still very warm.
While potatoes cool, add mayonnaise, chopped cilantro, and chopped red onion to medium bowl. Stir well until ingredients are thoroughly incorporated.
Once potatoes have cooled slightly, transfer mayo mixture to bowl with sweet potatoes. Gently stir, folding potatoes into mayo mixture until potatoes are thoroughly coated. Serve salad immediately or let cool to room temperature and then serve.