Classic Potato and Caramelized Onion, Gruyère Gratin
Experience this classic potato presentation—thinly sliced potatoes layered with a distinctive Mornay sauce (a Béchamel sauce with Gruyère cheese added). The addition of savory, caramelized onions and fresh thyme infuses comforting flavors into every bite.
Author: Cooks On Main
Servings: 6–8
View Original PDFIngredients
- 2 tablespoons butter
- 2 medium sweet onions, sliced pole to pole
- 1 teaspoon fresh thyme leaves
- 2 lbs Russet potatoes (about 2–3 medium to large), peeled and thinly sliced with a mandolin
- 3 cloves garlic, minced
Mornay Sauce Ingredients
- 4 tablespoons butter
- 1/4 cup all-purpose flour
- 3 cups whole milk
- 1 1/2 teaspoons Kosher salt (Try our Olivelle Roasted Garlic Sea Salt!)
- 1/4 teaspoon black pepper
- 2 cups grated Gruyère, 1/4 cup reserved for topping
Directions
- In a large skillet, melt 2 tablespoons butter and sauté onions over medium heat. Reduce heat to medium-low and cook slowly, stirring occasionally, for 35–45 minutes until caramelized. Add garlic and cook 2–3 minutes more. Stir in thyme.
- While onions cook, peel and rinse potatoes. Slice into 1/8-inch-thick rounds using a mandolin.
- Preheat oven to 400°F.
- In a small saucepan, melt 4 tablespoons butter. Add flour and whisk for 1–2 minutes until smooth to make a roux.
- Slowly add milk to roux while whisking. Add salt and pepper and cook over medium-low heat for 3–5 minutes until thickened.
- Remove from heat and whisk in 1 3/4 cups Gruyère until smooth. This is your Mornay sauce.
- Butter a baking dish. Layer potatoes, then onions, then sauce. Repeat layers until within 1/2 inch of top of dish.
- Top with remaining onions, sauce, and reserved 1/4 cup Gruyère.
- Cover with foil or lid and bake for 60 minutes. Uncover and bake an additional 10 minutes until golden and bubbly.
- Garnish with fresh thyme stems and serve hot.