Preheat oven to 350 degrees F.
Combine milk, 3/4 cup cheddar, 1/2 cup Monterey Jack, ketchup, Worcestershire sauce, and pepper; set aside.
Arrange half of potatoes and half of onion in the bottom of a 13 x 9 baking dish coated with cooking spray.
Top with half of milk mixture.
Repeat layers; top with 3/4 cup cheddar, 1/2 cup Monterey Jack, and parsley.
Cover and bake at 350 degrees for 1 hour and 15 minutes.
Uncover; bake an additional 20 minutes or until potatoes are tender and cheese is browned.