Preheat oven to 350ºF. Lightly spray a 9×13-inch pan with cooking spray.: You will notice a warm oven smell as the temperature rises, signaling the air is ready to transform the casserole from creamy to bubbling with a golden top. Ensure the pan is evenly coated so the baked edges do not stick, and place a rack in the center of the oven for even heat. A common mistake is preheating to the wrong rack position, which can cause uneven browning. If your oven runs hot or cold, use an oven thermometer to verify the actual temperature before baking.
In a large skillet cook ground beef over medium-high heat until no longer pink. Drain fat. Return meat to skillet. Add taco seasoning and water. Stir to combine. Cook for 5 minutes.: As the ground beef sizzles, you should hear a lively bubbling and see the meat darken to rich brown, releasing savory juices and browned bits that deepen flavor. Draining excess fat improves texture and ensures the sauce will cling to the meat rather than float. When you add the taco seasoning and water , smell the spices bloom, and the skillet will go from dry to glossy. Simmering for about five minutes helps the flavors meld and reduces raw spice notes. Avoid rushing this step, as undercooked seasoning can give a powdery taste. If your skillet is crowded, the meat will steam instead of brown, so work in batches if necessary.
Stir in soup, Velveeta and salsa. Reduce heat to low, and cook until the cheese melts, stirring constantly.: When you add the Cream of Chicken Soup , cubed Velveeta , and salsa , the mixture will look chunky at first, then gradually smooth into a glossy, cohesive sauce. Keep the heat low to prevent the dairy from separating, and stir gently in a slow figure eight to coax the cheese into a silky emulsion. You should see small ribbons of melted cheese disappear into the sauce, and the aroma will shift to rich, creamy tomato and spice notes. A frequent error is turning the heat too high which can cause the cheese to become grainy. If the sauce seems too thick, add a splash of water and stir until desired consistency is reached.
Stir in pierogies and pour into prepared dish. Top with cheddar cheese.: As you fold in the frozen pierogi , they will initially feel cold and a bit firm, then gradually warm and absorb some sauce. Gently toss to coat each one, being careful not to tear their delicate edges. Pouring the mixture into the prepared baking dish, distribute the pierogi evenly so every serving has a balance of dumpling to filling. Sprinkle the shredded cheddar cheese evenly on top to form a melting crust. An easy mistake here is packing the dish too tightly, which can prevent hot air circulation and lead to uneven baking. Leave a little space between pieces for the casserole to bubble and brown.
Bake for 30 minutes or until heated through.: During baking you will see the sauce gently bubble around the edges and the cheese on top turn from glossy to golden and slightly crisp. The aroma will shift toward toasted, savory cheese and warm spices, and you may hear gentle bubbling as moisture escapes. Bake until the interior is piping hot and the top is nicely browned, testing in the center with a fork for heat. Overbaking can dry out the pierogi so check a bit early if your oven browns quickly. If the top browns too fast, tent lightly with foil to finish heating without burning the cheese.