In a large skillet, melt the butter over medium heat. Add the thinly sliced onions and sauté until soft and translucent, about 5-7 minutes. Add in the minced garlic and cook for an additional minute until fragrant.
In a mixing bowl, combine the sautéed onions and garlic with the chopped cooked chicken, roasted red peppers, cream cheese, and if desired, 4 ounces of shredded Monterey jack cheese. Mix until well incorporated. Season with dried herbs or garlic powder if desired.
Spread a thin layer of green enchilada sauce on the bottom of your baking dish. Take a flour tortilla, place a generous scoop of the creamy chicken filling in the center, roll it up tightly, and place seam side down in the baking dish. Repeat until all filling is used.
Pour the remaining green enchilada sauce over the top, ensuring each tortilla is well-coated. Sprinkle the remaining 8 ounces of shredded Monterey jack cheese evenly over the sauce.
Preheat your oven to 350°F (175°C). Cover the baking dish with aluminum foil and bake for 20 minutes. Remove the foil and bake for an additional 10-15 minutes, or until the cheese is bubbly and golden.
