COOK macaroni as directed on package, omitting salt.
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 dressing. 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.
DRAIN macaroni. Add to sauce with bacon; mix lightly.