Preheat oven to 450 F. Place the tomatillos, jalapenos, garlic, and onion on a baking sheet. Bake for 20 minutes on the upper rack of the oven, turning vegetables half way through at 10 minutes. If you're cooking the chicken for this recipe and not using a rotisserie-cooked chicken (see notes), cook chicken while vegetables are roasting.
Remove roasted vegetables from oven and let cool slightly. Remove and discard dark, blistered skin from tomatillos. Lower oven heat to 350 F.
Blend cooked vegetables in a blender on low for a few seconds. Add cilantro leaves and salt to blender. Pulse a few times until blended. Mixture should be smooth but with seeds still visible and not completely blended. Add heavy cream-- 1/2 cup for a more tangy, acidic sauce, and 1 full cup for a creamier, richer sauce. Taste sauce and add more salt if needed. Add 3/4 cup of the sauce to the shredded chicken and mix together. Set both aside.
Heat a nonstick pan or skillet on medium-high heat. Drop 1 teaspoon of oil on the pan at a time and cook tortillas, front and back for a few seconds each, until there are golden brown spots on both sides. Add more oil as needed, every few tortillas.
Spray a 9 x 13 inch pan with cooking spray. Coat the bottom of the pan with sauce (1/2 - 1 cup). If using corn tortillas, you will make two rows of enchiladas side by side, and if using flour tortillas, you will make one long row of enchiladas down the length of the pan. Hold the tortilla in one hand, and with the other fill it with 1/4-1/3 cup of chicken and a sprinkle of cheese. Roll up the enchilada in the pan and position seam side down. Assemble the rest of the enchiladas. If you have extra tortillas and filling, use another small pan to make additional enchiladas. Pour the remaining sauce over enchiladas and spread evenly over the tops of enchiladas with a spatula. Sprinkle with remaining cheese. Spray a sheet of aluminum foil with cooking spray, and wrap pan with greased foil.
Bake enchiladas at 350 F for 20 minutes covered, and 10 additional minutes uncovered. During the last minute or two of cooking, turn the oven to broil, and cook until tops of enchiladas have some brown spots and are bubbly. Do not walk away from the oven during this process- your cheese can burn easily in seconds. Serve hot and garnish with fresh cilantro, chopped onions, or sour cream if desired.
*You may use pre-cooked tortillas in this recipe, but uncooked tortillas are highly recommended for optimal quality and flavor. Corn are preferred, but flour are delicious also. You will need more tortillas if using corn. We recommend buying a little extra in the event that you need more or have extra enchilada filling.**You can use any kind of cooked chicken for this recipe but we recommend using a cooking method that will keep the chicken moist. Either remove chicken from one full rotisserie-cooked chicken or cook 3-4 chicken breast halves. We love this instant pot method for cooking chicken breasts that are fresh or frozen in the Instant Pot.