Heat oven to 400 degrees. Put sweet potatoes and onions on a large baking sheet, drizzle with 2 tablespoons oil, toss to coat and spread out in a single layer. Sprinkle with salt and pepper. Roast, turning occasionally, until potatoes begin to brown on corners and are just tender inside, 30 to 40 minutes. Remove from oven; keep on pan until ready to mix with dressing.
Put chilies in a blender or mini food processor along with garlic, lime juice, remaining olive oil and a sprinkle of salt and pepper. Process until blended.
Put warm vegetables in a large bowl with beans and bell pepper; toss with dressing and cilantro. Taste and adjust seasoning if necessary. Serve warm or at room temperature, or refrigerate for up to a day.