Heat the oven to 350 degrees and prepare a 9 x 13-inch baking dish with cooking spray or lightly coated in oil.
Cook the Quinoa: In a small saucepan, combine the uncooked quinoa and chicken broth and bring to a boil. Reduce to a simmer and cook until the water is absorbed and the quinoa is tender, approximately 15 minutes. Remove from the heat and set aside.
Sauté the Vegetables: In a large skillet over medium-high heat, heat a tablespoon of olive oil. Add the onions and 1/2 teaspoon of salt and cook until tender, stirring occasionally, approximately 5 minutes. Add the Italian peppers, red pepper, chili powder, cumin, remaining 1/2 teaspoon of salt, and cook until the vegetables are barely softened, about five minutes. Add the garlic and cook for another 30 seconds until fragrant.
Add the Chicken, Beans, Chilies, and Sauce: Turn the heat down to medium, and add the rotisserie chicken, drained black beans, diced chilis, and enchilada sauce to the skillet. Cook for another 10 minutes to allow the flavors to blend. Add the chopped spinach and cook until barely wilted, about 30 seconds to one minute. Remove the pan from the heat.
Mix in the Quinoa and Cilantro & Spread in Prepared Dish: Add the cooked quinoa and chopped cilantro (if using) to the skillet and toss to combine. If you don't have enough room in the skillet, combine the quinoa and chicken mixture in a large bowl. Spread half of the mixture in the prepared baking dish and sprinkle with half of the cheese. Top with the other half of the quinoa chicken mixture and sprinkle the remaining cheese. Add more cheese if you prefer a super cheesy topping! Bake until the casserole is hot and the cheese is melted, approximately 20 to 25 minutes. Let sit for five minutes, and then top with desired garnishes to serve.