**Cook the Pasta Shells**
Preheat your oven to 375°F (190°C). Cook the jumbo pasta shells according to the package instructions until al dente. Drain and set aside to cool slightly. Be sure to save a bit of pasta water in case you need to thin the sauce.
**Cook the Ground Beef**
In a large skillet, cook the ground beef over medium-high heat, breaking it up as it cooks. Once the beef is browned, drain any excess fat. Stir in the taco seasoning, sour cream, and a pinch of salt and pepper. Cook for another 2-3 minutes until the mixture is well combined and heated through.
**Make the Filling**
In a large mixing bowl, combine the cooked beef mixture with the shredded cheddar cheese, shredded mozzarella cheese, and cream cheese. Stir until the mixture is creamy and well combined.
**Prepare the Sauce**
In a small bowl, mix the marinara sauce with the garlic powder and onion powder. If the sauce is too thick, add a little bit of water to thin it out to your desired consistency.
**Assemble the Stuffed Shells**
Spread a thin layer of the tomato sauce on the bottom of a 9x13-inch baking dish. Carefully stuff each cooked pasta shell with the beef and cheese mixture, then arrange them in a single layer in the baking dish. Pour the remaining tomato sauce over the stuffed shells, spreading it evenly.
**Bake the Stuffed Shells**
Cover the baking dish with aluminum foil and bake in the preheated oven for 25 minutes. After 25 minutes, remove the foil and sprinkle the remaining shredded cheddar cheese on top of the shells. Return the dish to the oven and bake for an additional 10 minutes, or until the cheese is melted and bubbly.
**Serve**
Once baked, remove the dish from the oven and let it cool for a few minutes. Serve the cheesy taco stuffed shells with your favorite toppings such as chopped green onions, cilantro, or jalapeños.