Preheat the oven to 350°F. Spray a 9x13-inch baking dish with nonstick cooking spray.
Add oil to a small skillet to a depth of about an inch. Heat over medium heat until the oil starts to shimmer. Fry the tortillas in the hot oil for 30 to 45 seconds each, turning once with tongs. The texture starts to look a bit leathery. Drain the tortillas on paper towels and cool enough to be easily handled.
In a mixing bowl, combine the chicken, 1 cup of the Jack cheese, the cream cheese, onion, and taco seasoning mix. Season the mixture to taste with salt and pepper.
Assemble the enchiladas by placing 2 tablespoons filling on each tortilla. Roll and place the enchiladas, seam side down, in the prepared baking dish.
Pour the salsa over the enchiladas and sprinkle the reserved cheese over the enchiladas. Bake the enchiladas for 20 minutes, or until the dish is heated through and the cheese is melted.
To freeze: Wrap the dish after step 4 and chill in the refrigerator before freezing. Place the remaining cheese in a small zip-top, freezer bag and store both in the freezer for up to two months. To cook, thaw completely in the refrigerator before proceeding with step 5.