2poundsmoderate to high starch potatoes (see blog post for more info), thinly sliced
1/2cupfinely chopped yellow onion
1/2cupheavy cream(use full fat)
1 1/2cupsshredded Cheddar cheese(another fabulous option is half Gruyere/half Emmentaler)
1/4cupgrated Parmesan cheesedivided in half
Preheat the oven to 350 degrees F.
Melt the butter in a stock pot over medium-high heat. Add the flour and whisk until smooth. Simmer, whisking constantly, for 2 minutes. Add the onions and garlic and stir to combine. Add the milk, cream and salt and whisk until smooth. Bring to a simmer and whisk until thickened. Whisk in the Cheddar and half of the Parmesan cheese. Whisk until melted. Stir in the potatoes.
Pour the potato mixture into a 2 quart casserole dish. Sprinkle with remaining Parmesan cheese.
Cover with foil and bake for 1 hour or until the potatoes are soft. Remove foil and bake for another 20 minutes or until the top is lightly browned. Let sit for 5 minutes before serving.
*Make Ahead: See section in blog post about making this dish ahead of time.