Preheat grill to medium high heat.
While grill is heating, slice the peppers and onions into thin strips.
Heat a small skillet on the stove or over the grill and add 1 tablespoon butter to the skillet. When melted, add the peppers and onions and cook about 5 minutes, stirring often, until tender crisp.
Shape beef into 4 patties and sprinkle each side with taco seasoning. Place on grill and cook for 15 minutes, or until cooked to your liking, flipping halfway through cooking.
Top burgers with cheddar cheese and cook 2 minutes more to melt.
Remove burgers from grill and let rest 5 minutes.
While burgers are resting, spread remaining butter evenly between the buns and place on grill to toast.
Place burgers on toasted buns and top with peppers, onions, cilantro, and avocado slices.
Squirt sour cream on to the top of each bun and top the burgers. Serve immediately.