A detailed recipe for cheesy scalloped potatoes featuring Yukon Gold potatoes, a rich white sauce made with whole milk, butter, onion, garlic, and flour, finished with Gruyere and Parmesan cheeses.
1. Place the sliced potatoes in a large pot and add the milk and 1 tsp kosher salt. Mix and bring to a simmer over high heat. Once the milk starts bubbling (8-10 mins), turn the heat down to low and simmer for 5 mins, stirring occasionally. Remove the potatoes from the milk with a slotted spoon and let cool. Reserve the milk for the sauce and clean out the pot.
2. Preheat the oven to 375F.
3. Return the pot to the stove over medium heat. Melt the butter and then add the onions with a sprinkle of salt and pepper. Cook the onions while stirring for 3-5 mins until they are starting to become translucent, then add the garlic and flour.
4. Cook for 1 min, stirring constantly, then start gradually adding the milk 1/2 cup at a time, stirring so that all the milk gets fully incorporated before adding more. Once all of the milk is mixed in, add 3/4 tsp kosher salt and 3/4 tsp pepper and bring to a simmer over medium high heat. Once the white sauce begins to bubble and thickens slightly (3-4 min), turn off the heat. Taste and adjust seasoning if desired.
5. Add a layer of sauce to a 9x13” (or similar) baking dish, followed by a layer of potatoes, a generous sprinkle of gruyere, and a generous sprinkle of parmesan. Repeat 2-3 more times until all potatoes are used, ending in white sauce and cheese on top.
6. Cover with foil and place on a baking sheet. Bake for 45 mins covered, then remove the foil and bake for an additional 25-30 mins uncovered until potatoes are bubbly and the top is golden brown.
7. Let rest for 5 mins, sprinkle with parsley, and enjoy!
