Preheat oven and prep pan: Preheat the oven to 425°F and line a large baking sheet with parchment paper for easy cleanup.
Season the sweet potatoes: In a large bowl, toss the cubed sweet potatoes with olive oil, cumin, chili powder, smoked paprika and salt until evenly coated.
Roast until caramelized: Spread the sweet potatoes in a single layer on the prepared baking sheet and roast for 25 to 30 minutes, flipping halfway, until the edges turn deep golden and the centers are fork-tender.
Cook the quinoa: While the sweet potatoes roast, combine the quinoa and water in a small saucepan, bring to a boil, then reduce heat to low, cover and simmer for 15 minutes until the liquid is absorbed.
Fluff the quinoa: Remove the quinoa from heat and let it sit covered for 5 minutes, then fluff with a fork before setting aside.
Warm the black beans: In a small saucepan over medium-low heat, warm the black beans with a splash of water for about 5 minutes until heated through and glossy.
Prep the toppings: Dice the avocado, thinly slice the red onion, chop the cilantro and squeeze the lime juice while the other components finish cooking.
Warm the corn: If using frozen or canned corn, warm it briefly in a dry skillet over medium heat for 2 to 3 minutes until heated through.
Assemble the bowls: Divide the cooked quinoa among four bowls, then top each with roasted sweet potatoes, black beans and corn.
Finish and serve: Top each bowl with diced avocado, feta cheese, red onion and cilantro, then drizzle with lime juice and a dollop of Greek yogurt if using.