Preheat oven to 350°.
Combine milk, 3/4 cup cheddar, 1/2 cup Monterey Jack, ketchup, Worcestershire, and pepper; set aside.
Arrange half of potatoes and half of onion in the bottom of a 13 x 9-inch 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° for 1 hour and 15 minutes. Uncover; bake an additional 20 minutes or until po- tatoes are tender and cheese is browned.