Step 1: Prepare the Baking Dish - Preheat your oven to 350 degrees Fahrenheit. Lightly coat a 9x13 inch baking dish with nonstick cooking spray so the sauce and cheese around the edges release more easily when you serve the shells. Getting the pan ready first is one of those small things that makes assembly feel much smoother once the pasta and filling are finished.
Step 2: Cook and Cool the Pasta Shells - Bring a large pot of salted water to a boil and cook the jumbo shells according to the package directions. You want them tender enough to eat but still sturdy enough to hold the filling, so avoid cooking them beyond the recommended time. Once they are done, drain the shells and rinse them gently under cold water. The cold water stops the cooking process and, just as importantly, makes the shells cool enough for you to handle without burning your fingers while you stuff them. A few shells may tear while boiling. That happens. I usually cook the full package amount called for so I have enough good shells to fill the baking dish without having to wrestle with the broken ones.
Step 3: Brown the Ground Beef - While the pasta cooks, place the ground beef in a large skillet over medium heat. Break it into small crumbles with a wooden spoon or spatula as it cooks. Continue cooking until there is no pink remaining and the beef is evenly browned. Drain away the excess grease before moving on. Since cream cheese is going into the filling, getting rid of that extra grease keeps the final mixture creamy rather than heavy or oily.
Step 4: Season the Beef - Return the drained beef to the skillet and sprinkle in the taco seasoning. Add the amount of water called for on your seasoning packet, since brands can vary a little. Stir everything together and let the beef simmer for about 2 to 3 minutes. You are looking for the liquid to reduce and the seasoning mixture to thicken around the beef. Instead of watery meat sitting in the bottom of the skillet, you should see a glossy, seasoned coating clinging to every little crumble.
Step 5: Make the Creamy Taco Filling - Add the cream cheese to the seasoned beef, then turn off the heat. Stir while the beef is still hot so the cream cheese melts into the mixture. At first you may see little streaks or chunks of cream cheese. Keep stirring. Within a minute or two, it should become a smooth, creamy filling with the beef evenly distributed throughout. This is the part that takes ordinary taco meat and makes it especially good inside pasta shells. You want a filling that is soft enough to spoon easily but thick enough to stay tucked inside each shell.
Step 6: Sauce and Stuff the Shells - Spoon a few generous spoonfuls of enchilada sauce into the prepared baking dish and spread it across the bottom. You do not need a deep pool of sauce. You just want enough to give the shells a moist, flavorful base while they bake. Pick up one cooled jumbo shell and gently open it with your fingers. Spoon a heaping tablespoon of the beef and cream cheese filling into the center, then place the stuffed shell into the baking dish with the opening facing up. Continue filling and arranging the shells until you have used the filling. You can nestle them fairly close together. I actually like having them snug in the pan because they support one another and make the finished casserole easy to portion.
Step 7: Add the Sauce and Cheese - Pour the remaining enchilada sauce over the stuffed shells. Try to spread it across the pan so every section gets some sauce, especially along the exposed edges of the pasta. Sprinkle the shredded fiesta blend cheese evenly over the top. A cup gives you a nice cheesy covering, while 1 1/2 cups creates that thicker, gooier top layer that tends to disappear quickly at my table.
Step 8: Bake Until Hot and Melty - Place the baking dish in the preheated 350 degree oven and bake for about 15 minutes. The filling is already cooked, so you are mainly warming everything through, letting the sauce settle around the shells, and melting the cheese. When the cheese is fully melted and the sauce is hot around the edges, pull the pan from the oven. Give the shells a few minutes to sit before serving. That short rest makes them easier to lift from the dish and gives the filling a chance to settle inside the pasta.