Preheat the oven to 350°F (175°C).
In a large skillet over medium heat, add the 1.5 pounds lean ground beef and the diced onion. Cook, breaking the meat into pieces, until the beef is cooked through and the onion is softened and translucent.
Drain any excess grease from the skillet. Return the skillet to the stove and stir in the 2 tablespoons taco seasoning until evenly combined with the meat and onion. Remove from heat and set aside.
Pour 1/4 cup of the 19 ounces red enchilada sauce into the bottom of a 9×13-inch baking dish and spread it to coat the bottom.
Heat a small skillet over medium heat and add about 1 tablespoon of the 3 tablespoons cooking oil. When the oil is hot, place one tortilla in the skillet and warm for about 10–20 seconds per side, flipping once, until pliable and lightly coated with oil. Repeat with the remaining tortillas, adding additional oil from the remaining cooking oil as needed (total oil used should not exceed the 3 tablespoons).
Place a warmed tortilla on a plate or cutting board, spoon a portion of the beef mixture down the center (use roughly the same amount for each tortilla so they are easy to roll), and roll tightly. Place each rolled tortilla seam-side down in the prepared baking dish. Continue until all 12 tortillas are filled and arranged in the dish.
Pour the remaining enchilada sauce evenly over the rolled enchiladas so they are covered.
Sprinkle the 1 cup shredded cheddar cheese evenly over the sauced enchiladas.
Bake in the preheated oven for 20–25 minutes, or until the cheese is melted and the sauce is bubbling at the edges.
Remove from the oven and garnish with freshly chopped cilantro and/or pico de gallo as desired. Serve hot.