Heat oven to 400ºF. Line a sheet pan with foil. Spread sweet potato on the sheet pan, drizzle with 2 tablespoons of olive oil and sprinkle with 1 teaspoon cumin and chili powder. Toss until evenly coated. Bake in the oven for 45 minutes or until potatoes are tender. While potatoes roast in the oven, heat remaining 1 tablespoon of olive oil in a large nonstick skillet. Add onions and garlic, sauté until fragrant, about 5 minutes. Add black beans, remaining cumin, and lime juice. When potatoes are done, add them to the beans and mix until well combined. Combine avocado, crema, cilantro, garlic, salt, and lime juice in a blender. Blend until smooth. Serve sweet potato mixture in warm tortillas, drizzled with avocado crema.