First (bottom) layer: mix together beans, salsa, cumin, and garlic powder, and spread on large, flat serving dish as a thin layer, maybe 1/2 inch thick.
Second (middle) layer: mix together sour cream and taco seasoning, and spread over bean layer.
Third (top) layer: sprinkle the cheese on first, then top with chopped tomatoes and green onions.
Cover with plastic wrap, and chill for about an hour. Serve cold with tortilla chips.