Preheat oven to 400F degrees.
Melt butter in skillet. Add onion and cook until tender and translucent.
Add beans to the skillet. Mash until the beans and onions reach desired texture.
Add sour cream, green chilies, and taco seasoning. Mix until fully combined. Taste and adjust ingredients to desired flavor.
Top with Cheddar cheese and bake until cheese is melted. Garnish with ciced Green onion. Serve.