Meanwhile, melt butter in large saucepan on medium heat. Add onions; cook and stir 2 min. or until golden brown. Stir in flour; cook 2 min., stirring constantly. Whisk in milk, then Miracle Whip
. Bring just to simmer, stirring constantly; cook and stir 2 to 3 min. or until thickened. Add cheeses and Sriracha sauce; cook and stir 2 min. or until shredded cheese is melted.