Brown 1 pound hamburger with 1 small chopped onion and set aside. Melt 1 (12 oz.) bag shredded cheddar cheese with 1 pound Velveeta cheese in saucepan on low heat or in microwave, stirring every 2 minutes. Combine cheese and hamburger. Add 2 cans green chilies, chopped and 1/2 jar mild taco sauce. Serve warm as dip for tortilla chips.