Preheat oven to 375 degrees. Line greased 15x10-inch baking pan with parchment paper; grease paper and set aside.
In large bowl, beat cream cheese and milk until smooth. Add flour, salt, pepper and mustard; mix until smooth. Add eggs and blend well. Pour into prepared pan and place carefully into oven.
Bake for 25 to 30 minutes or until eggs are puffed and set. Remove from oven, sprinkle with 1 cup of cheese, ham, spinach, mushrooms and onions. Top with remaining cheese, reserving ¼ cup to sprinkle on top of roll.
Roll omelet from short side, peeling parchment paper away while rolling. Sprinkle top of roll with remaining cheese; bake an additional 3 to 5 minutes or until cheese is melted.