Begin by steaming the cauliflower florets until tender, about 8-10 minutes. You want them soft enough to mash but not mushy.
In a large skillet, heat the extra-virgin olive oil over medium heat. Add the chopped onion and minced garlic, cooking until the onion is translucent and fragrant, about 5 minutes.
In a mixing bowl, combine the steamed cauliflower, sautéed onion and garlic, shredded chicken, taco seasoning, diced green chilis, red enchilada sauce, large egg, egg white, salt, and black pepper. Mix well until everything is evenly coated.
Preheat your oven to 375°F (190°C). Grease your baking dish with a little olive oil. Spread the cauliflower mixture evenly in the dish, then sprinkle the reduced-fat Mexican cheese blend on top.
Bake in the preheated oven for about 25-30 minutes or until the cheese is melted and bubbly, and the edges are slightly golden.
Remove the casserole from the oven and let it cool for a few minutes. Top with fresh cilantro, diced tomatoes, and sliced avocado before serving.