Prepare the shredded chicken: add the chicken thighs, diced onion, 15-oz can green enchilada sauce, 1 cup green salsa or chicken broth, 4-oz can diced green chiles, and 1 tsp sea salt to your Instant Pot or chosen cooking pot; stir to combine.
If using an Instant Pot, secure the lid and pressure cook on High for 20 minutes, then allow a natural release for at least 10 minutes before quick-releasing remaining pressure; if using slow cooker or stovetop, cook until chicken is fully cooked and tender (see your preferred method).
Use tongs to transfer the cooked chicken to a cutting board and shred it with two forks.
Return the shredded chicken to the pot and mix with the cooking juices; reserve about 1/2 cup of the liquid for assembling the casserole.
Preheat the oven to 400°F (204°C) and lightly grease a 13 x 9-inch casserole dish if needed.
Pour the reserved 1/2 cup of chicken liquid into the bottom of the prepared casserole dish; if using leftover chicken without sauce, use canned green enchilada sauce or extra salsa for this step.
Dip each tortilla briefly into the shredded chicken mixture so it absorbs some sauce (this softens them); if you prefer drier tortillas, skip the dipping.
Place about a heaping 1/2 cup of shredded chicken down the center of each tortilla, roll tightly, and arrange seam-side down in the casserole dish; repeat until all 8 tortillas are filled and packed in the dish.
Evenly spread 1 to 2 cups green salsa (or additional enchilada sauce) over the assembled enchiladas, then sprinkle the 2 cups grated jack cheese on top.
Bake in the preheated oven for about 20 minutes, until the sauce is bubbly and the cheese is melted and lightly golden.
Remove from the oven and, if desired, sprinkle with chopped green onions, cotija cheese, and fresh cilantro before serving.
