Preheat the oven to 350 degrees F (175 degrees C). Butter a baking pan.

Heat a pan over medium-high heat. Add chicken and saute until no longer pink in the center and juices run clear, 6 to 8 minutes. Drain excess liquid. Add taco seasoning; toss to coat. Transfer chicken into the prepared baking pan.

Mix salsa, sour cream, and jalapenos together in a bowl and pour over the chicken.

Bake in the preheated oven until hot, about 25 minutes. Remove from oven and cover with Cheddar cheese. Continue baking until cheese melts, about 5 minutes more.

Let casserole cool before cutting into pieces and garnishing with green onions.