Preheat your oven to 375°F and lightly grease a small baking dish.
In a small saucepan over medium heat, combine the tomato puree, chili powder, cumin, garlic powder, onion powder, sea salt, and black pepper to create the ranchero sauce.
Heat the olive oil in a skillet over medium heat and sauté the diced white onion until translucent and soft.
In a mixing bowl, combine the shredded cooked chicken breast with the sautéed onions and 2 tablespoons of the prepared ranchero sauce.
Warm the corn tortillas slightly in a dry pan or microwave to make them pliable, then fill each with the chicken mixture and roll tightly.
Place the tortillas seam-side down in the baking dish, pour the remaining ranchero sauce over the top, and sprinkle with shredded cheddar cheese.
Bake for 15 to 20 minutes until the sauce is bubbling and the cheese has melted beautifully.
Garnish with freshly chopped cilantro before serving.