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, pepper, garlic powder, onion soup mix, monterey jack cheese, and half of the cheddar cheese (reserve the rest for topping).
Spread potatoes into the casserole dish. Top with remaining cheese.
Bake for 30 minutes. Add the onions and bake for an additional 10 minutes.
Notes
Freezer Friendly: Prepare casserole as instructed and use a disposable pan. Leave off the fried onions. Cover and store in the freezer. To bake, thaw in the fridge and bake as instructed.