"Old Rotten" Potatoes

When I was a kid, I misheard "au gratin" as "old rotten". I thought the name was a bit off-putting, but damn were old rotten potatoes good.

Ingredients

• 3 lbs yukon gold or red potatoes
• 1 small yellow onion, diced
• 6 cloves garlic, minced
• 8 oz gruyere or sharp chedder (or 4 oz of each)
• 1/4 C flour
• 1 tsp yellow mustard
• 4 tbsp butter
• 2 C milk

Preparation

Preheat the oven to 400°F. Generously butter a 9x13" baking dish.

Use a mandoline to thinly slice potatoes. Collect these thin slices in stacks onto a plate so it will be easy to assemble later.

Melt butter in a saucepan over medium heat. Add the garlic and cook until fragrent, about 30 seconds. Add the mustard and flour and whisk continuously until the smell of raw flour is gone, about a minute.

Add milk slowly, whisking vigorously until its well combined and smooth. Bring to a simmer, whisking periodically. Remove from heat. Mix in cheese and whisk until smooth and melted.

Sprinkle the bottom of the baking dish with half the onions. Pour about a cup of the cheese sauce into the bottom of the dish. Arrange the potatoes atop the cheese sauce evenly in layers. Sprinkle the remaining onions on top and pour all the cheese sauce over the top. Smooth out the cheese sauce as necessary.

Bake for 60-75 minutes until the top is browned and a knife slides easily into the center of the potatoes without resistance. Let cool for 10 minutes before serving.