Preheat your oven to 375°F (190°C).
Poach the chicken breast in simmering water for 12-15 minutes until cooked through, then shred using two forks.
Heat olive oil in a skillet over medium heat and sauté diced onions and bell peppers until tender.
Add minced garlic, chili powder, and cumin to the skillet, stirring for 1 minute until fragrant.
In a small bowl, combine the tomato puree with sea salt and black pepper to create the ranchero sauce.
Briefly warm the corn tortillas in a dry pan or microwave to make them pliable.
Fill each tortilla with the shredded chicken and sautéed vegetable mixture, then roll tightly.
Place the tortillas seam-side down in a small oven-safe baking dish.
Top the tortillas evenly with the ranchero sauce and sprinkle with shredded cheddar cheese.
Bake for 15-20 minutes until the sauce is bubbling and the cheese has melted completely.
Garnish with freshly chopped cilantro before serving warm.