Prep. Preheat your oven to 425°F and line a baking sheet with parchment paper.
Season. In a bowl, combine the sweet potato, olive oil, salt, garlic powder, smoked paprika, and cumin. Toss to coat the sweet potatoes evenly.
Roast. Spread the sweet potatoes in a single layer on the prepared baking sheet and roast for 30 minutes, stirring halfway through.
For the avocado lime ranch dressing
Blend. Add the avocado, olive oil, mayonnaise, sour cream, buttermilk, lime juice, apple cider vinegar, dill, cilantro, garlic powder, onion powder, and cumin to a blender, and blend until smooth and creamy.
Adjust. Taste the dressing and season it with salt and pepper if desired. If the dressing is too thick, blend in more buttermilk, 1 tablespoon at a time, until you reach your desired consistency.
To assemble the salad
Put it all together. Toss together the lettuce greens, radishes, cherry tomatoes, corn, black beans, toasted pepitas, and roasted sweet potatoes. Top with avocado and crumbled queso fresco.
Dress. Pour the dressing over the salad and toss gently to coat. Serve.