Description
This French Onion Potato Gratin combines the rich, caramelized flavors of French onions with tender, creamy sliced russet potatoes baked in a luscious blend of Gruyere and Parmesan cheeses. The dish is finished with a golden, bubbly cheese topping, making it a perfect comforting side or centerpiece for a cozy meal.
Ingredients
For the Caramelized Onions
- 2 Tablespoons unsalted butter
- 1 pound yellow onion, thinly sliced
- ½ teaspoon kosher salt
- ¼ teaspoon black pepper
- 2 sprigs fresh thyme
- 2 cloves garlic, minced
- ½ cup beef broth
- 1 teaspoon balsamic vinegar
For the Gratin
- 3 pounds russet potatoes, peeled and cut into 1/8 inch slices
- 1 ½ cups shredded Gruyere cheese (about 6 ounces)
- ½ cup freshly grated Parmesan cheese (about 2 ounces)
- 2 sprigs fresh thyme, leaves removed
- ¾ cup heavy cream
Instructions
- Caramelize Onions: Melt the butter in a 12-inch heavy-bottomed skillet over medium heat. Add the sliced onions, thyme sprigs, kosher salt, and black pepper. Cook, stirring frequently, until the onions are very soft and have developed a deep golden-brown color, about 25 to 30 minutes. Adjust the heat as necessary to avoid burning, stirring more often if needed.
- Deglaze Pan: Add the minced garlic to the caramelized onions and cook for 1 minute until fragrant. Pour in the beef broth and scrape up any browned bits from the bottom of the pan. Bring the broth to a boil, then reduce heat and simmer until the liquid has reduced by half. Remove from heat, discard the thyme sprigs, and stir in the balsamic vinegar.
- Preheat Oven and Prepare Dish: Preheat the oven to 400°F (200°C). Grease a 12-inch casserole dish or a cast iron pan thoroughly with butter.
- Mix Cheese and Thyme: In a small bowl, combine the shredded Gruyere cheese, grated Parmesan cheese, and fresh thyme leaves.
- Combine Potatoes and Onion Mixture: In a large bowl, gently combine the sliced potatoes, caramelized onion mixture, heavy cream, and about two-thirds of the cheese mixture. Break up the potato slices slightly to ensure each slice is coated with the creamy onion-cheese mixture.
- Assemble the Gratin: Arrange the potato slices vertically in tight sideways stacks in the prepared baking dish. The stacks should be close enough to support each other; if not, slice an additional potato and add more slices. Spoon any remaining cheese and onion mixture from the bowl evenly over the potatoes, then pour any remaining cream over the top.
- Bake Covered: Cover the baking dish with foil and place it in the preheated oven. Bake for 60 minutes, allowing the potatoes to cook through and the flavors to meld.
- Broil with Cheese Topping: Remove the foil and evenly sprinkle the remaining cheese mixture over the gratin. Broil for 5 to 6 minutes until the cheese is bubbly and browned in spots, watching carefully to prevent burning.
- Rest and Serve: Remove the gratin from the oven and let it rest for 15 minutes to set. Garnish with additional fresh thyme if desired, then serve warm.
Notes
- Using thinly sliced potatoes (about 1/8 inch) ensures even cooking and tender texture.
- Be patient while caramelizing onions to develop deep, rich flavor without burning.
- The gratin can be prepared a day ahead up to assembling; refrigerate and bake before serving.
- For a vegetarian version, substitute vegetable broth for beef broth.
- Leftovers reheat well in the oven covered with foil to maintain moisture.
- Prep Time: 30 minutes
- Cook Time: 1 hour 40 minutes
- Category: Side Dish
- Method: Baking
- Cuisine: French