• Preheat oven to 325 ℉.
• Grease casserole dish generously with butter or non-stick spray. Set aside.
• Cut onion into thin slices.
• Peel russet potatoes and cut them into thin slices.
• Cut Yukon Gold potatoes into thin slices. Peel russet potatoes before slicing them. A mandolin makes quick work of slicing all the potatoes and the onion.
• Place the potato slices in a large bowl and cover with cold water. Set aside while you make the cheese sauce.

Make the cheese sauce:

• In a large sauce pan melt 3 tablespoons butter over medium heat.
• Once butter is melted add flour to the pan.
• Whisk flour and butter together to make a roux. Add salt and cook for another 2 minutes whisking continuously to prevent burning and to cook off the raw flour taste.
• Slowly add milk, whisking it into the roux.
• Whisk continuously until the sauce starts to thicken, about 2-3 minutes.
• Add 1½ cups of shredded cheddar cheese to the sauce and continue to whisk until the cheese has completely melted and you have a smooth, thick and creamy sauce. Remove pan from the heat.

Assemble casserole:

• Drain potatoes and pat with a paper towel to remove excess water.
• Place half the potato slices in the prepared casserole dish in an even layer and season with salt and pepper.
• Top with a layer of sliced onions, using all the onions.
• Place remaining potato slices in a even layer over the sliced onions, and season with salt and pepper.
• Pour the cheese sauce over the sliced potatoes and onions in the casserole dish.
• Top with an even layer of shredded cheddar cheese using all the remaining cheese.
• Spray a sheet of foil with non-stick cooking spray and cover the casserole dish with the foil.

Cook the casserole:

• Bake for 2-2½ hours or until the potatoes are tender when pierced with a sharp knife.
• Remove the foil and return casserole dish to the oven. Set oven temperature to "broil" and cook until the cheese on top is bubbly and golden brown along the edges.
• Remove from the oven and allow casserole to rest for 15-20 minutes before serving to allow everything to set up.