Preheat the oven to 350°F (175°C).
In a large mixing bowl, combine the cooked quinoa, thawed corn, 1 cup of the diced tomatoes, drained black beans, chopped purple onion, diced jalapeño (if using), enchilada sauce, and 1½ cups of the shredded cheddar. Stir until evenly mixed.
Spoon the mixture into a lightly greased 2-quart baking dish, spreading it evenly.
Top the casserole with the remaining 1½ cups of shredded cheddar cheese. Sprinkle half of the chopped cilantro, the remaining diced tomatoes, and the sliced black olives over the cheese.
Bake in the preheated oven for 40–50 minutes, until the top is bubbly and the cheese is melted.
Remove from the oven and let cool a few minutes. Garnish with the remaining cilantro and the diced avocado. Season with salt and pepper to taste, if desired, and serve.