Pre-heat oven to 350 degrees F. Lightly grease a 9x13 casserole dish with cooking spray, and set aside.
In a large bowl, mix together potatoes, soups, sour cream, seasoning, monterey jack cheese, and half of the cheddar cheese (reserve the rest for topping).
Spread potatoes into the casserole dish. Top with remaining cheese. In a sealable bag, toss cornflakes in melted butter. Evenly spread the cornflakes over the top of the casserole.
Bake for 45 minutes. The cornflakes will brown and casserole will be bubbly.