In a large skillet over medium heat, add 1/4 cup cooking oil. Whisk in 1/4 cup all-purpose flour, 3 tablespoons chili powder, 2 teaspoons cumin, 2 teaspoons garlic powder, 1 teaspoon oregano, and 1/2 teaspoon salt. Cook, stirring regularly, for 1–2 minutes to toast the spices and cook the flour.

Slowly pour in the 3 cups chicken broth in a thin stream while whisking constantly so the mixture becomes smooth. Bring to a simmer, reduce heat to low, and simmer 5–10 minutes, whisking occasionally, until the sauce has thickened enough to coat the back of a spoon. Remove from heat.

Preheat the oven to 350°F (175°C).

Pour 1/4 cup of the enchilada sauce into a 9×13-inch (or similar) casserole dish and tilt the dish so the sauce evenly coats the bottom.

Heat a small skillet over medium heat and add 1 tablespoon of the 3 tablespoons cooking oil. One at a time, briefly warm each of the 12 corn tortillas in the hot oil, flipping quickly so each side is lightly coated and the tortilla becomes pliable (add the remaining oil in portions as needed; use the total 3 tablespoons). Stack warmed tortillas and keep covered with a clean kitchen towel to stay warm and pliable.

Assemble the enchiladas: place a warmed tortilla on a flat surface, add 2–3 tablespoons shredded sharp cheddar cheese and about 1 teaspoon diced yellow onion in the center, then roll tightly and place seam side down in the prepared casserole dish. Repeat with all 12 tortillas. Reserve about 1 cup of the shredded cheese for the topping.

Pour the remaining enchilada sauce evenly over the rolled enchiladas so they are well coated. Sprinkle the reserved shredded cheese evenly over the top.

Bake uncovered for 20–25 minutes, or until the cheese is fully melted and the edges begin to bubble.

Remove from the oven and garnish with freshly chopped cilantro and/or the finely diced onion. Serve hot.