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A close-up features a golden-brown, bubbly surface of freshly baked French Onion Potatoes Au Gratin, showcasing its creamy texture and caramelized onion topping.

ULTIMATE FRENCH ONION POTATOES AU GRATIN

This French Onion Potatoes Au Gratin recipe elevates the classic dish with the rich, savory flavors of French onion soup. Layers of thinly sliced potatoes, sweet caramelized onions, Gruyère cheese, and a creamy sauce create a comforting and impressive side dish perfect for any occasion.
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Prep Time 30 minutes
Cook Time 1 hour 5 minutes
Total Time 1 hour 35 minutes
Servings: 6
Course: Side Dish
Cuisine: American, French
Calories: 550
Ingredients Equipment Method Notes

Ingredients
  

  • 2-3 pounds Yukon Gold potatoes, peeled and thinly sliced
  • 3-4 large yellow onions, thinly sliced
  • 1/2 cup butter
  • 4 cups beef broth
  • 2 cups heavy cream
  • 2 cloves garlic, minced
  • 2 tablespoons fresh thyme leaves
  • 8 ounces Gruyère cheese, shredded
  • Salt and pepper to taste
  • Pinch of nutmeg
  • 1 teaspoon sugar optional

Equipment

  • Large Skillet
  • Sharp Knife or Mandoline
  • Mixing bowl
  • Paper towels
  • 9x13 inch baking dish
  • Saucepan
  • Aluminum foil
  • Oven

Method
 

  1. Preheat oven to 375°F (190°C).
  2. Melt butter in a large skillet over medium heat. Add onions and a pinch of salt. Cook, stirring occasionally, for 30-45 minutes, or until deeply golden brown and caramelized.
  3. Stir in garlic and thyme. Cook for 1 minute until fragrant.
  4. Pour in beef broth and scrape up any browned bits from the bottom of the skillet. Simmer for a few minutes until broth has reduced slightly. Remove from heat.
  5. While onions are caramelizing, peel and thinly slice potatoes. Place in a bowl of cold water to prevent browning. Drain and pat dry.
  6. Grease a 9x13 inch baking dish with butter.
  7. Arrange a layer of potato slices in the bottom of the dish, slightly overlapping.
  8. Spoon about half of the caramelized onions over the potatoes.
  9. Sprinkle with about 1/3 of the Gruyère cheese.
  10. Repeat the layers: potatoes, onions, cheese.
  11. For the final layer, arrange the remaining potato slices on top and sprinkle with the remaining Gruyère cheese.
  12. In a saucepan, heat heavy cream over medium heat. Season with salt, pepper, and nutmeg. Bring to a simmer, then remove from heat.
  13. Pour cream sauce evenly over the potatoes in the baking dish.
  14. Cover the baking dish with aluminum foil.
  15. Bake for 45 minutes.
  16. Remove the foil and bake for another 15-20 minutes, or until the potatoes are tender and the cheese is melted and bubbly.
  17. Let the au gratin rest for 10-15 minutes before serving.

Notes

Don't skip the caramelization step for the onions. Use a mandoline for even slicing. Keep potato slices thin. Season generously. Use freshly grated cheese. Leftovers can be stored in the refrigerator for up to 3 days and reheated in the oven or microwave. Variations include adding extra garlic, red pepper flakes, different cheeses (Swiss, Parmesan), crispy bacon crumbles, or using vegetable broth for a vegetarian version. Experiment with herbs like rosemary, sage, or oregano. Assemble the au gratin ahead of time and refrigerate for up to 24 hours before baking.