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Top-down view of charred leek and potato gratin with golden gruyere crust in a white ceramic baking dish

Smoky Charred Leek and Potato Gratin with Gruyere Crust

Yukon gold potatoes and cast iron-charred leeks baked in garlic-thyme cream with a golden gruyere crust.
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Prep Time 20 minutes
Cook Time 1 hour 10 minutes
Total Time 1 hour 40 minutes
Servings: 6 servings
Course: Side Dish
Cuisine: French
Calories: 480
Ingredients Method Notes

Ingredients
  

For the Leeks and Potatoes
  • 4 leeks large (about 900 g); white and light green parts only, halved lengthwise, rinsed
  • 2 tbsp olive oil
  • 900 g Yukon gold potatoes sliced 3mm thick
  • 1 tsp kosher salt plus more to taste
  • 1/2 tsp black pepper freshly ground
For the Cream
  • 1 tbsp unsalted butter for greasing the dish
  • 3 cloves garlic minced
  • 500 ml heavy cream
  • 250 ml whole milk
  • 1 tbsp fresh thyme leaves only
  • 1 pinch nutmeg freshly grated
  • 200 g gruyere cheese grated
  • 50 g parmesan cheese grated

Method
 

Char the Leeks
  1. Halve leeks lengthwise, rinse any grit from between the layers, and pat dry.
  2. Heat a dry cast iron skillet over medium-high heat. Add leeks cut-side down with the olive oil and cook 8 to 10 minutes until deeply charred in spots, turning once.
  3. Set leeks aside to cool slightly, then roughly chop into 2-inch pieces.
Make the Cream
  1. In a saucepan over medium heat, combine cream, milk, garlic, thyme, and nutmeg. Bring to a gentle simmer, about 4 to 5 minutes, then remove from heat.
Assemble and Bake
  1. Heat the oven to 190 C / 375 F. Butter a 9x13 inch baking dish.
  2. Layer a third of the potato slices in the dish, overlapping slightly, and season with salt and pepper. Scatter half the charred leeks over top.
  3. Repeat with another layer of potatoes and the remaining leeks, then finish with a top layer of potatoes.
  4. Pour the warm cream mixture evenly over the layers, pressing down gently so it seeps between the slices.
  5. Scatter gruyere and parmesan over the top and cover tightly with foil.
  6. Bake covered for 35 minutes. Remove the foil and bake 20 more minutes, until the top is deep golden and a knife slides through the potatoes with no resistance.
  7. Rest 10 minutes before slicing and serving.

Notes

  • Dry the charred leeks well before layering, excess moisture makes the gratin watery.
  • Don't skip simmering the cream, it seasons the potatoes as they bake.
  • Test doneness with a paring knife through the center, not just the edges.
  • Use a shallow wide dish, not deep, for even cooking and more browned surface.