Follow these detailed instructions to create a perfect Spring Veggie Gratin every time. Each step is designed to ensure flavorful vegetables and a flawlessly creamy and cheesy sauce:

1. Preheat the Oven and Prepare the Baking Dish: Preheat your oven to 375°F (190°C). While the oven is preheating, lightly grease a 9×13 inch baking dish. This prevents the gratin from sticking and makes serving easier. A glass or ceramic baking dish works best for even heat distribution.
2. Prepare the Vegetables: Bring a large pot of salted water to a boil. Blanch the asparagus and peas separately. For the asparagus, boil for 2-3 minutes until bright green and slightly tender-crisp. Immediately transfer to an ice bath to stop the cooking process and preserve their vibrant color and crisp texture. For the peas, boil for 1-2 minutes until bright green and tender. Drain and also transfer to the ice bath. Once cooled, drain both vegetables thoroughly. This blanching step ensures the vegetables are tender-crisp in the final gratin and retain their bright spring colors. Set aside the blanched asparagus and peas. Drain and quarter the artichoke hearts and set them aside as well.
3. Sauté the Leeks and Garlic: Heat olive oil in a large skillet over medium heat. Add the sliced leeks and sauté for 5-7 minutes, or until softened and translucent. Stir frequently to prevent browning and ensure even cooking. Add the minced garlic and cook for another minute until fragrant, being careful not to burn the garlic. Sautéing the leeks and garlic first mellows their flavors and creates a flavorful base for the gratin. Remove the skillet from the heat and set aside.
4. Make the Béchamel Sauce: In a separate medium saucepan, melt the butter over medium heat. Once melted, whisk in the all-purpose flour and cook for 1-2 minutes, stirring constantly, to create a roux. This step is crucial for thickening the sauce and preventing lumps. The roux should be smooth and pale golden in color. Slowly whisk in the milk, a little at a time, ensuring each addition is fully incorporated before adding more. Continue whisking constantly to prevent lumps from forming and ensure a smooth sauce. Once all the milk is added, bring the sauce to a simmer, stirring constantly, and cook for 5-7 minutes, or until the sauce has thickened slightly and coats the back of a spoon. The sauce should be smooth and creamy.
5. Flavor the Béchamel Sauce: Remove the saucepan from the heat. Stir in the heavy cream, grated nutmeg, salt, and black pepper to taste. Taste and adjust seasoning as needed. The heavy cream adds richness and the nutmeg provides a warm, aromatic note. Seasoning at this stage is important to ensure a flavorful sauce.
6. Combine Vegetables and Sauce: Add the sautéed leeks and garlic, blanched asparagus and peas, and quartered artichoke hearts to the béchamel sauce in the saucepan. Gently stir to combine all the vegetables with the creamy sauce, ensuring they are evenly coated.
7. Assemble the Gratin: Pour the vegetable and sauce mixture into the prepared baking dish, spreading it out evenly. Sprinkle the grated Gruyere cheese and Parmesan cheese evenly over the top of the vegetable mixture. The cheese topping will create a golden brown and bubbly crust in the oven.
8. Bake the Gratin: Bake in the preheated oven for 25-30 minutes, or until the gratin is bubbly, golden brown on top, and heated through. The cheese should be melted, bubbly, and lightly browned. If the top is browning too quickly, you can loosely tent it with foil for the last 10 minutes of baking.
9. Garnish and Serve: Remove the gratin from the oven and let it rest for a few minutes before serving. Sprinkle with fresh thyme leaves for garnish and added freshness. Serve hot and enjoy the delightful flavors of spring!