HEAT oven to 425ºF.
MIX first 4 ingredients until well blended; spread onto tortillas. Place 1 tortilla in 13x9-inch dish sprayed with cooking spray; top with 1/4 each of the cheese, corn and onions. Repeat layers 3 times; cover.
BAKE 25 to 28 min. or until heated through.
TOP with tomatoes and sour cream. Cut into wedges.