Preheat your oven to 400°F (200°C). On a large baking sheet, combine the diced sweet potatoes with olive oil, chili powder, cumin, garlic powder, salt, and black pepper. Toss well to ensure the sweet potatoes are evenly coated. Spread them out in a single layer for optimal roasting.

Roast the seasoned sweet potatoes for 20-25 minutes, flipping them halfway through. Continue roasting until they are tender when pierced with a fork and have achieved a lovely light caramelization on the edges.

While the sweet potatoes are roasting, prepare the black beans. In a small saucepan, combine the rinsed and drained black beans, vegetable broth, cumin, chili powder, and a pinch of salt. Heat over medium-low heat for 5-7 minutes, stirring occasionally, until the beans are warmed through and the flavors have melded. For a creamier texture, you can mash some of the beans lightly with a fork.

In a small bowl, whisk together the Greek yogurt, fresh lime juice, chopped cilantro, minced garlic, 1 tablespoon of water, salt, and black pepper until the dressing is smooth and well combined. If the dressing is too thick, add a little more water, 1 teaspoon at a time, until it reaches your desired consistency.

To assemble the taco bowls, divide the cooked brown rice or quinoa (if using) evenly among 4 serving bowls. Top each bowl with equal portions of the roasted sweet potatoes, spiced black beans, thawed corn, finely diced red onion, and diced avocado.

Generously drizzle each bowl with the creamy lime-cilantro dressing. Garnish with additional fresh cilantro. Serve immediately, adding any optional toppings such as cherry tomatoes, sliced jalapeños, crumbled cotija cheese, or tortilla chips for extra crunch.