Preheat your oven to 375°F and lightly grease a small baking dish with a touch of oil.
In a small skillet, heat the olive oil over medium heat and sauté the diced yellow onion until soft and translucent.
Stir the cooked shredded chicken breast into the skillet with the onions and add 2 tablespoons of the tomato puree to keep the meat moist and flavorful.
In a separate bowl, whisk together the remaining tomato puree, ancho chili powder, ground cumin, garlic powder, sea salt, and black pepper to create the enchilada sauce.
Warm the corn tortillas slightly in a dry pan or microwave for 15 seconds so they are pliable and do not crack when rolling.
Dip each tortilla into the ancho sauce to coat both sides, then fill with the chicken and onion mixture and roll tightly.
Place the rolled tortillas seam-side down in the baking dish and pour the remaining sauce evenly over the top.
Sprinkle with shredded Monterey Jack cheese and bake for 15 minutes until the sauce is bubbly and the cheese is melted.
Serve the enchiladas warm, topped with a dollop of Greek yogurt and fresh avocado slices.