Preheat oven to 400F. Place the prepared sweet potatoes on a baking sheet. Drizzle with oil and toss to coat. Sprinkle with salt to taste. Roast for 30 minutes or until golden and lightly crispy.
Meanwhile, heat oil in a large skillet over medium heat. Add the onion and bell pepper and sauté for 5-10 minutes or until softened. Stir in the black beans, cumin, chili powder and salt and cook for another 2 minutes. Remove from heat.
Prepare the dressing: In a high-speed blender or small food processor, add all the dressing ingredients. Blend for 30 seconds.
Assemble the salad. Place the leafy greens and tomatoes in a large bowl, along with the roasted sweet potatoes and cooked onion, pepper and beans. Drizzle with the dressing and toss to combine.
Serve into 3-4 bowls, and top with avocado and green onions.