Put milk, shredded cheddar cheese, water, butter, and salt in a large pot and stir. Bring to a boil over medium heat, stirring often.
Remove from heat and whisk in instant potato flakes.
Stir in beaten eggs.
Spread into a greased 9x9" baking dish.
Bake at 350˚F for 30-35 minutes, until puffy and golden.
Video
Notes
To make the day before, prepare the potatoes but cover and refrigerate before baking. The next day, remove the pan from the refrigerator a 1/2 hour before baking.