Poach the chicken breast in simmering water until cooked through, then shred with two forks.
In a small skillet, heat olive oil over medium heat and sauté the diced onion and minced garlic until fragrant.
Stir in the tomato puree, chili powder, cumin, salt, and pepper to create the ranchero sauce; simmer for 5 minutes.
Lightly warm the corn tortillas in a dry pan until pliable.
Dip each tortilla into the ranchero sauce, fill with shredded chicken, and roll tightly.
Place the rolled tortillas in a small baking dish, top with the remaining sauce and shredded cheddar cheese.
Bake at 375°F for 10-12 minutes until the cheese is bubbly and edges are crisp.
Garnish with fresh chopped cilantro before serving.