Preheat the oven to 400°F (200°C). Spray a 9x13-inch baking dish with non-stick cooking spray.
Wash, peel and slice the potatoes into ¼-inch slices and arrange half of the potato slices into the prepared baking dish.
In a large skillet or saucepan, melt the butter over medium heat. Add the sliced onions and saute until soft and translucent. Stir in the flour, salt, and black pepper and continue cooking for another 1-2 minutes.
Whisk in the heavy cream and bring to a simmer over medium heat. Cook until smooth and thickened.
Reduce the heat to low and stir in the cheddar cheese, continue cooking until the cheese is fully melted and smooth. Stirring constantly.
Pour half of the cheese sauce over the arranged potato slices and spread evenly. Arrange the other half of the potato slices over top and pour on the remaining cheese and spread evenly.
Cover the dish with foil and bake for 90 minutes, or until the potatoes are tender.
After baking, remove the foil from the dish and set your oven to broil.
Broil the dish uncovered for 5-10 minutes or until golden brown.
Allow to cool for 10 minutes, before serving.
Garnish with fresh chopped green onions and serve. Enjoy!