Preheat your oven to 375°F and lightly grease a small baking dish with a spray of avocado oil.
Season the chicken breast with a pinch of salt and pepper, then poach in simmering water for 12-15 minutes until cooked through; remove and shred with two forks.
In a medium skillet, heat 1 teaspoon of avocado oil over medium heat and sauté the diced yellow onion and red bell pepper until they are soft and slightly caramelized.
In a small mixing bowl, combine the tomato puree, ancho chili powder, ground cumin, and garlic powder, whisking until the sauce is smooth and well-incorporated.
Briefly warm the corn tortillas in a dry skillet for 30 seconds per side until they become soft and pliable.
Spread a thin layer of the ancho sauce on the bottom of the baking dish to prevent sticking.
Dip each tortilla into the sauce, fill with a portion of the shredded chicken and sautéed vegetables, then roll tightly and place seam-side down in the dish.
Pour the remaining sauce over the top of the enchiladas and sprinkle evenly with the shredded Monterey Jack cheese.
Bake for 15 to 20 minutes until the cheese is melted and the sauce is bubbling around the edges.
Remove from the oven and finish with a sprinkle of fresh cilantro before serving.