Preheat oven to 400 degrees F.
Toss sweet potato and 1 tsp. oil on a large rimmed baking sheet. Roast, stirring once halfway through, until tender, about 25 minutes.
Meanwhile, combine the remaining 1 1/2 tsp. oil, avocado, lime juice, garlic, cumin, salt, pepper, and 1 Tbsp. water in a blender or food processor; process until smooth. Add 1 Tbsp. water, if needed, to reach desired consistency.
Combine the sweet potato, quinoa, black beans, and kale in a medium bowl. Drizzle with the avocado dressing and gently toss to coat. Top with pepitas and scallion.
Pepitas (hulled pumpkin seeds) can be found in the bulk-foods section of natural-foods stores and Mexican groceries.
To make ahead
Prepare sweet potato (Steps 1-2) and dressing (Step 3). Refrigerate separately for up to 2 days.