Creating this Creamy Leek and Pea Gratin is easier than you might think! Follow these detailed steps for a foolproof and delicious result:

1. Prepare the Leeks: Begin by thoroughly cleaning the leeks. Leeks often trap dirt and grit between their layers, so this step is crucial. To clean them, trim off the dark green tops and the root end. Slice the white and pale green parts lengthwise, then rinse them under cold running water, separating the layers to remove any dirt. Once cleaned, thinly slice the leeks crosswise.
2. Sauté the Leeks and Garlic: In a large skillet or Dutch oven, melt 2 tablespoons of butter over medium heat. Add the sliced leeks and minced garlic. Sauté the leeks, stirring occasionally, until they are softened and translucent, about 8-10 minutes. Be careful not to brown them; you want them to be tender and sweet. Sautéing the leeks gently draws out their natural sweetness and mellows their flavor, creating a delicious base for the gratin. The garlic adds a subtle aromatic layer.
3. Make the Roux: Reduce the heat to low. Add the remaining 2 tablespoons of butter to the skillet with the softened leeks. Once the butter is melted, sprinkle in the all-purpose flour. Cook the flour and butter together, stirring constantly with a whisk, for about 1-2 minutes. This mixture, called a roux, is the foundation of your creamy sauce. Cooking the roux slightly cooks out the raw flour taste and ensures a smooth sauce.
4. Create the Creamy Sauce: Gradually whisk in the milk, a little at a time, ensuring each addition is fully incorporated before adding more. This prevents lumps from forming and creates a smooth sauce. Once all the milk is added, whisk in the heavy cream. Continue to whisk and cook the sauce over low heat until it thickens slightly, about 5-7 minutes. The sauce should be thick enough to coat the back of a spoon. Gentle heat is key here to prevent scorching the milk and cream.
5. Incorporate the Cheese and Seasoning: Remove the skillet from the heat. Stir in the grated Gruyere cheese and Parmesan cheese, stirring until the cheeses are melted and the sauce is smooth and cheesy. The residual heat from the sauce will melt the cheese beautifully. Season the sauce with salt, black pepper, fresh thyme (or dried thyme), and a pinch of ground nutmeg. Taste and adjust seasoning as needed. Don’t be shy with the seasoning; cheese and cream can handle a good amount of salt and pepper. The nutmeg adds a warm, subtle spice that complements the creamy richness.
6. Add the Peas: Gently stir in the thawed frozen peas. Distribute them evenly throughout the creamy cheese sauce. You want to add the peas at the end to prevent them from becoming mushy during baking. They will heat through and retain their vibrant color and slightly sweet texture.
7. Prepare the Breadcrumb Topping: In a small bowl, combine the panko breadcrumbs with a drizzle of olive oil (optional, but it helps them crisp up nicely) and a pinch of salt. Toss to coat the breadcrumbs evenly. You can also melt a tablespoon of butter and toss the breadcrumbs in melted butter for extra richness and flavor.
8. Assemble and Bake the Gratin: Preheat your oven to 375°F (190°C). Pour the creamy leek and pea mixture into a greased 9×13 inch baking dish or a similar sized oven-safe dish. Sprinkle the prepared breadcrumb topping evenly over the surface of the gratin.
9. Bake Until Golden and Bubbly: Bake in the preheated oven for 25-30 minutes, or until the gratin is bubbly around the edges and the breadcrumb topping is golden brown and crisp. The baking time may vary slightly depending on your oven and the depth of your baking dish. You want the gratin to be heated through and nicely browned on top.
10. Rest and Serve: Remove the gratin from the oven and let it rest for 5-10 minutes before serving. This allows the sauce to set slightly and prevents it from being too runny. Resting also makes it easier to serve and prevents burning your mouth!