Make the sauce by pulsing the jalapeno and garlic in a food processor or blender to mince, then adding the remaining sauce ingredients and processing to combine. Set aside.
Preheat oven to 425 degrees F. In a large skillet, melt the butter over medium heat, then add the onion and bell pepper. Season with a little salt and pepper and saute until soft, about 5 minutes. Add the green chilies during the last minute.
In a large bowl, combine the sauteed onions and peppers with the cooked, shredded chicken and ½ cup of the pepperjack cheese. Season with a little salt and pepper, to taste. Mix well.
Warm the tortillas for 45 seconds in the microwave until warm and pliable, then fill each tortilla with chicken mixture and roll them up. Spread 1 cup of the pumpkin enchilada sauce in the bottom of a large rectangular baking dish, then place the rolled tortillas in the sauce, seam side down. Cover with the remaining sauce and cheese, then bake for 25-30 minutes, until the cheese is melted and the enchiladas are hot.
Sprinkle with the green onions, chopped cilantro, and avocado, and serve.