Awaken the Aromatics: Preheat your oven to 425°F (220°C). Place the Dutch oven over medium-high heat and add the olive oil. Once shimmering, add the diced onion and chopped jalapeño. Sauté for about 4-5 minutes, stirring occasionally, until they begin to soften.
Bloom the Spices: Stir in the smoked paprika, ancho chile powder, and dried oregano. Cook for another 45 seconds until the spices are deeply fragrant. Add the minced garlic and cook for 30 seconds more, stirring constantly to prevent scorching.
Build the Base: Add the rinsed quinoa to the pot and stir for one minute to lightly toast the grains. Pour in the low-sodium vegetable broth and the fine sea salt. Stir in the drained fire-roasted tomatoes and the corn.
Simmer the Grains: Bring the mixture to a steady simmer. Once simmering, reduce the heat to low, cover the pot tightly with its lid, and let it cook for 18-20 minutes. The quinoa is done when all the broth has been absorbed and the grains are tender.
Fold in Fillings: Remove the pot from the heat. Gently fold in the rinsed black beans and 1 cup of the shredded cheese blend until evenly distributed throughout the quinoa mixture.
Bake to Perfection: Smooth the top of the mixture into an even layer. Sprinkle the remaining ¾ cup of cheese uniformly over the surface. Transfer the uncovered Dutch oven to the preheated oven. Bake for 10-12 minutes, or until the casserole is heated through and the cheese topping is fully melted and bubbly.
Garnish and Serve: Carefully remove the Dutch oven from the oven. Let it rest for 5 minutes before garnishing generously with the fresh cilantro and scallions. Serve hot with lime wedges for squeezing and a dollop of sour cream or Greek yogurt, if desired.