HEAT oven to 400ºF.
CRUMBLE chorizo into large skillet; cook on medium heat 12 min. or until done, stirring frequently. Drain; return to skillet. Add onions; cook 3 min., stirring occasionally. Stir in hash browns and sour cream.
SPOON into 13x9-inch baking dish sprayed with cooking spray; top with cheese. Cover.
BAKE 40 to 45 min. or until heated through.