Start by rinsing the quinoa under cool water. In a medium saucepan, combine 1 cup of rinsed quinoa with 2 cups of water. Bring it to a boil, then reduce the heat to low, cover, and let simmer for about 15 minutes, or until the quinoa is fluffy and the water is absorbed. Set aside.
In a large skillet, heat 1 tablespoon of extra-virgin olive oil over medium heat. Add the chopped yellow onion and sauté until translucent, about 3-4 minutes. Then, add the green and red bell peppers, cooking until they are tender, about 5 minutes.
Add the bite-sized chicken pieces to the skillet with the sautéed vegetables. Season with 1 tablespoon + 2 teaspoons of chili powder, 1 teaspoon of ground cumin, kosher salt, ground black pepper, cayenne pepper, and minced garlic. Cook, stirring frequently, until the chicken is cooked through and no longer pink, about 7-10 minutes.
Once the chicken is cooked, stir in the well-drained fire-roasted diced tomatoes and rinsed black beans. Mix in the cooked quinoa and half of the cheddar and mozzarella cheeses. Stir until everything is well combined.
Preheat your oven to 350°F (175°C). Transfer the chicken and quinoa mixture into a greased 9x13 inch baking dish. Spread it out evenly, and then dollop the plain Greek yogurt over the top. Sprinkle the remaining cheddar and mozzarella cheeses evenly across the casserole.
Place the baking dish in the preheated oven and bake for 25-30 minutes, or until the cheese is melted and bubbly. The edges should be slightly golden brown.
Once done, remove the casserole from the oven and let it sit for about 5 minutes. Garnish with fresh cilantro before serving. Enjoy your hearty, flavorful Mexican Chicken Casserole!
