Mother Cloutier's Potatoes au Gratin

Preheat oven to 350°F (175°C). Cube or dice the potatoes into uniform pieces approximately ½-inch in size, then place them in a pot of salted water.

Bring the pot to a boil over medium-high heat, then reduce to a simmer and cook the potatoes until they are just tender but still hold their shape, approximately 8-10 minutes.

Drain the potatoes thoroughly and set aside. In a large saucepan, melt the butter over medium heat, then add the chopped onion and cook until softened and fragrant, about 3-4 minutes.

Sprinkle the flour over the butter and onion mixture, stirring constantly to create a roux. Cook for 1-2 minutes until the flour is incorporated and the mixture is smooth.

Gradually add the milk while stirring constantly to prevent lumps from forming. Continue stirring over medium heat until the sauce thickens, approximately 5-7 minutes.

Tear or cut the American cheese slices into pieces and stir them into the hot sauce until completely melted and smooth. Season with salt and pepper to taste if desired.

Gently fold the drained potatoes into the cheese sauce until evenly coated. Transfer the mixture to a buttered baking dish.

Bake in the preheated 350°F oven for 25-30 minutes until the top is lightly golden and the dish is heated through.

Remove from the oven and let rest for 5 minutes before serving. The gratin will continue to set slightly as it cools.