Preheat oven to 350˚F.: You will notice the oven warming and the kitchen getting a subtle dry heat, which ensures a steady environment for baking; preheating matters because even gentle ovens can cause uneven melting if you start with a cold pan, and a common mistake is placing the dish in before it reaches temperature which can lengthen bake time and affect cheese melt.
In a large skillet, cook the ground turkey and chopped onion over medium heat. Break the meat up as it cooks until it is cooked through and no longer pink. Drain any excess grease and mix in the taco seasoning. Once done, remove from heat.: Expect the sound of gentle sizzling as the onion softens and the ground turkey browns, releasing savory juices; the aroma shifts from raw to rich and toasted, signaling flavorful Maillard notes; stirring and breaking the meat helps even browning which deepens taste, and draining excess grease prevents a greasy casserole; a typical slip is undercooking the meat or leaving it wet, which can make the filling watery, so cook until the meat is fully opaque and the onion is translucent.
Once done, remove from heat.: You will feel the pan cool slightly and the immediate aroma will relax; removing the skillet from heat stops carryover cooking, preserving texture and preventing the filling from becoming dry when baked, and a misstep is leaving it on the heat too long where flavors can become overly concentrated and the mixture may clump.
Add 1 tablespoon of oil to a small skillet over medium heat. Once the oil is hot, add the tortillas and flip each side to quickly warm and coat with oil. Repeat with all tortillas, adding additional oil to the skillet as needed.: The oil should shimmer and make the tortillas lightly crisp at the edges, the quick toasting brings out a nutty corn scent and makes the shells pliable for rolling; this technique prevents tearing when you roll and keeps the interior from becoming mushy, and the usual error is overheating the oil which will blister or burn the tortillas, so watch for a quick bubble and pull them as soon as they warm.
Pour 1/4 cup of the enchilada sauce in the bottom of a large baking dish. I used a 9×13 inch casserole. Spread to coat the bottom.: The sauce will hiss slightly when it meets the warm dish and forms a thin flavorful layer that prevents sticking and infuses the bottom of the tortillas; this base layer ensures each enchilada bakes into saucy tenderness, and skipping this step often leads to dry bottoms and uneven flavor distribution.
Fill each tortilla with a spoonful of meat mixture and roll tightly placing seam side down in the baking dish. Repeat until all of the turkey filled enchiladas are rolled and in the pan.: Rolling should feel snug but not forced, the warm tortilla will soften around the savory filling and create a neat seam that holds during baking; seam side down keeps them intact and encourages even sauce coverage, and a common oversight is overfilling which causes bursting during bake, so use measured spoonfuls for uniform results.
Pour the remaining enchilada sauce over the enchiladas so that they are all covered in sauce, then sprinkle the cheese over the top.: Pouring slowly lets the sauce settle into gaps and coat the tops and edges, you will see the sauce pool and then mellow into the tortillas; sprinkling the cheddar cheese evenly ensures a golden, bubbly finish, and uneven sauce can create dry spots while too much cheese can overpower the tang of the sauce, so aim for balanced coverage.
Bake for 20-25 minutes or until the cheese is melted and the edges start to bubble. Garnish with your choice of chopped cilantro or pico de gallo.: During baking you will hear gentle bubbling and smell toasted cheese and tomato aromatics, the edges will bronse and the top will shimmer when done; this baking stage melds flavors and firms the filling, and a frequent mistake is underbaking which leaves cheese unmelted and sauce cold, so look for even bubbling and a lightly golden top before removing from oven.