Step 1: Rinse 1 cup of quinoa thoroughly under cold running water. In a medium saucepan, combine the rinsed quinoa with 2 cups of vegetable broth or water. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes, or until all liquid is absorbed. Fluff with a fork and set aside.
Step 2: While quinoa cooks, preheat your oven to 400°F (200°C). Dice the sweet potatoes into 1/2-inch cubes. On a baking sheet, toss the sweet potatoes with 1 tablespoon of olive oil, salt, and pepper. Roast for 20-25 minutes, or until tender and slightly caramelized, flipping halfway through.
Step 3: In a large skillet or Dutch oven, heat the remaining 1 tablespoon of olive oil over medium heat. Add the diced yellow onion and sauté for 5-7 minutes until softened and translucent. Add the minced garlic, cumin, and smoked paprika, cooking for an additional minute until fragrant.
Step 4: Stir the fresh spinach into the skillet with the onion and spices. Cook for 2-3 minutes, stirring occasionally, until the spinach has completely wilted down.
Step 5: In a large mixing bowl, combine the cooked quinoa, roasted sweet potatoes, sautéed onion and spinach mixture, and the rinsed black beans. Stir in 1/2 cup of vegetable broth and 1/4 cup of chopped fresh cilantro. If using, gently fold in 1/2 cup of shredded cheese. Season with salt and pepper to taste.
Step 6: Lightly grease a 9×13 inch baking dish. Transfer the casserole mixture into the prepared dish, spreading it evenly. If desired, sprinkle the remaining 1/2 cup of shredded cheese over the top for a golden crust. Cover the dish loosely with aluminum foil.
Step 7: Bake the covered casserole for 20 minutes. Remove the foil and continue to bake for another 10-15 minutes, or until the cheese is melted and bubbly, and the casserole is heated through and slightly golden on top.
Step 8: Remove from the oven and let the Baked Sweet Potato Quinoa Casserole rest for 5-10 minutes before serving. Garnish with extra fresh cilantro if desired. Serve warm and enjoy!