Preheat your oven to 375°F (190°C).
In a small saucepan over low heat, combine the tomato puree, ancho chili powder, minced garlic, ground cumin, dried oregano, sea salt, and black pepper; simmer for 5 minutes to allow the flavors to meld.
In a non-stick skillet, sauté the diced white onion and bell pepper with a splash of water until they are tender and slightly translucent.
In a mixing bowl, toss the sautéed vegetables with the cooked shredded chicken breast until evenly combined.
Warm the corn tortillas in a dry pan for 30 seconds per side or wrap them in a damp towel and microwave for 20 seconds until they are soft and pliable.
Spread a thin layer of the ancho sauce on the bottom of a small baking dish.
Dip each tortilla lightly into the sauce, fill with the chicken and vegetable mixture, roll tightly, and place seam-side down in the prepared baking dish.
Pour the remaining sauce over the top of the enchiladas and bake for 15 minutes until the sauce is bubbling and hot.
Remove from the oven and garnish with crumbled cotija cheese, sliced avocado, and fresh cilantro before serving.