Preheat the oven to 350°F and have ready a 9x13" casserole dish.
Add oil to a large skillet over medium-high heat and allow it to come to temperature. Add the onion and sauté, stirring often, until softened, about 4 minutes.
Add the ground beef or turkey to the skillet and sprinkle over the taco seasoning. Break apart with a spatula and cook for about 10-12 minutes, stirring often, until fully cooked.
Add the beans (with the chili sauce), tomato sauce, salt, and garlic powder to the pan and stir until everything is well combined. Remove from the heat.
Place the taco shells upright in the casserole dish, and divide the meat mixture evenly amongst the shells. Mix the Cheddar and Monterey Jack together in a small bowl, and add a generous tablespoon of cheese to the top of each taco.
Transfer to the oven and bake for about 10 minutes, or until the cheese is completely melted and the taco shells are warmed through. Remove the pan from the oven and allow to cool slightly.
Have ready your favorite taco toppings, serve, and enjoy!