To the bottom of the casserole dish, add a layer of potatoes, a thin layer of onions, a sprinkle of cheddar cheese, and a small sprinkle of parmesan, repeating until the ingredients are used up. We make about 5-6 layers, ending with cheese on the top.
To a medium saucepan, add cream**, thyme, salt and pepper. Bring to a boil and remove from heat. Remove thyme. Carefully pour over the potatoes.
Cover with foil and bake for 1 hour. After 1 hour, remove foil and bake 20-30 minutes until potatoes are tender and cooked through and cheese is browned.
Allow to rest for 10 minutes before serving.
Notes
*We sometimes instead line the pan with parchment paper for easy cleanup.**For a lower fat and lower calorie version, in place of cream, melt 3 Tablespoons butter. Whisk in 3 Tablespoons flour and cook for 2 minutes. While whisking continuously, slowly add 2 cups skim milk or broth (chicken or vegetable broth). Add thyme, salt, and pepper. Bring to a boil and boil for 1 minute.