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Au Gratin Potatoes

Au gratin potatoes with French-inspired thyme cream sauce and thin, tender layers baked until bubbling and golden. This method makes a thickened milk-and-cream béchamel-style sauce for classic creamy slices.
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Prep Time 25 minutes
Cook Time 1 hour 15 minutes
Resting 15 minutes
Total Time 1 hour 55 minutes
Servings: 8 servings
Course: Side Dish
Cuisine: French-inspired
Calories: 650
Ingredients Equipment Method Notes

Ingredients
  

Yukon gold potatoes
  • 3 lb Yukon gold potatoes peeled and sliced 1/8 inch thick
  • 3 butter
Sauce
  • 3 tbsp butter
  • 3 clove garlic minced
  • 3 tbsp all-purpose flour
  • 2 cup whole milk
  • 1 cup heavy cream
  • 1.5 tsp salt
  • 0.5 tsp black pepper
  • 1 tsp fresh thyme leaves
  • 1 cup shredded sharp cheddar divided
  • 1 cup shredded Gruyere divided

Equipment

  • 1 sheet pan

Method
 

Preheat and prepare the baking dish
  1. Preheat the oven to 375F and butter a 9x13-inch baking dish so the potatoes release easily after baking.
Make the creamy sauce base
  1. Melt the butter, then cook the minced garlic for 30 seconds until fragrant.
  2. Whisk in the all-purpose flour and cook for 1 minute to remove the raw flour taste.
  3. Slowly whisk in the whole milk and heavy cream, then simmer until the sauce thickens enough to coat the back of a spoon.
  4. Season the thickened sauce with salt, black pepper, and fresh thyme leaves, stirring until evenly distributed.
  5. Remove the pan from the heat and stir in 1 cup of shredded sharp cheddar and 1/2 cup of shredded Gruyere until melted and smooth.
Layer and bake the gratin
  1. Layer a third of the sliced Yukon gold potatoes in the buttered dish, then spoon over a third of the sauce.
  2. Repeat the potato-and-sauce layering two more times, distributing the sauce so the top layer stays creamy.
  3. Top with the remaining cheddar and Gruyere for a golden, bubbling crust.
  4. Cover with foil and bake for 45 minutes so the potatoes soften through.
  5. Uncover and bake 25-30 minutes at 375F until the top is golden and the potatoes are tender.
Rest and serve
  1. Rest the au gratin potatoes for 15 minutes before serving so the sauce thickens and slices hold their shape.

Notes

Pro tip: slice the potatoes to a consistent 1/8 inch thickness so every layer finishes at the same time and stays creamy instead of watery. Refrigerate leftovers in a sealed container for up to 3 days; reheat covered in a 325F oven until hot. Freezing is not recommended because the cream can break after thawing. For a dairy-friendly option, use lactose-free milk and lactose-free cream plus a lactose-free cheese blend with similar melt.