Preheat your oven to 375°F (190°C).
In a medium skillet over medium heat, add the olive oil and sauté the diced onion and bell pepper until softened and slightly caramelized.
In a mixing bowl, combine the cooked shredded chicken breast with the sautéed vegetables, sea salt, black pepper, and garlic powder.
Warm the corn tortillas in a dry skillet or microwave for 15 seconds until they are soft and pliable.
Place a portion of the chicken mixture into each tortilla, roll them up tightly, and place them seam-side down in a small baking dish.
Pour the ranchero sauce evenly over the tortillas and sprinkle with the shredded cheddar cheese.
Bake for 15 minutes, or until the sauce is bubbling and the cheese has melted completely.
Top with a dollop of Greek yogurt and a sprinkle of fresh cilantro before serving.