HEAT oven to 375ºF.
PREPARE Dinner as directed on package. Meanwhile, combine shredded cheeses. Whisk egg, milk and seasonings until well blended.
HEAT oil in small skillet on medium-high heat. Add onions; cook and stir 2 min. or until crisp-tender. Add to Dinner; mix well. Spoon half the Dinner mixture into greased 2-qt. casserole; top with half the shredded cheese mixture. Repeat layers. Drizzle with egg mixture.
BAKE 15 min. or until center is set and Dinner mixture is heated through.