Mushroom and Leek Gratin
By America's Test KitchenPublished on October 19, 2021
Time
1½ hours
Yield
Serves 6 to 8
Ingredients
Before You Begin
Leeks come in a variety of sizes. If possible, purchase ones that have more white than green. If you can find only predominately green leeks, then buy closer to 5 pounds of leeks to ensure that you have enough to yield the 10 cups of sliced leeks needed for this recipe. To make vegetarian: You can substitute vegetable broth for the chicken broth, if desired.
Instructions
- Adjust oven rack to middle position and heat oven to 450 degrees. Melt 1 tablespoon butter in 12-inch nonstick skillet over medium heat. Add panko and ¼ teaspoon salt and cook, stirring frequently, until golden brown, about 5 minutes. Transfer to bowl; set aside. Wipe skillet clean with paper towels.
- Melt 2 tablespoons butter in now-empty skillet over medium heat. Add leeks (skillet may be full), water, and ½ teaspoon salt. Cover and cook until leeks are tender, 7 to 9 minutes. Uncover and continue to cook until water has evaporated, about 2 minutes longer. Transfer leeks to 13 by 9-inch baking dish and spread in even layer; set aside.
- Melt remaining 2 tablespoons butter in now-empty skillet over medium-high heat. Add cremini mushrooms, shiitake mushrooms, pepper, and remaining ¾ teaspoon salt. Cover and cook until mushrooms have released their moisture, about 4 minutes. Uncover and continue to cook, stirring occasionally, until mushrooms are deep golden brown and tender, 6 to 8 minutes longer.
- Stir in flour, garlic, and thyme and cook for 1 minute. Stir in cream and broth and bring to boil. Off heat, stir in Parmesan until smooth. Pour mushroom mixture over leeks in baking dish.
- Transfer to oven and bake until bubbling around edges, 5 to 7 minutes. Transfer dish to wire rack and let cool for 10 minutes. Sprinkle with toasted panko. Serve.TO MAKE AHEAD: At end of step 4, let casserole cool completely. Cover dish with aluminum foil and refrigerate for up to 24 hours. Store panko in airtight container at room temperature. To serve, bake, covered, for 30 to 35 minutes.
Time
1½ hoursYield
Serves 6 to 8Ingredients
Test Kitchen Techniques
Ingredients
Test Kitchen Techniques
Ingredients
Test Kitchen Techniques
Why This Recipe Works
For a streamlined vegetarian holiday gratin, we pared down the vegetable options to earthy mushrooms and subtly sweet leeks. To avoid a watered-down mess, we started by precooking both vegetables to draw out their moisture. Next we built a sauce of heavy cream and chicken broth (or vegetable broth for a true vegetarian version) to tie them together. We seasoned the sauce with garlic and thyme, thickened it with flour, and finished it with a hefty dose of grated Parmesan. Then we combined the sauce and vegetables in a baking dish and popped it into a 450-degree oven until it was bubbling and warmed through. To finish, we sprinkled buttery pretoasted bread crumbs over the gratin after it had cooled for 10 minutes. Pretoasting the bread crumbs ensured a crispy top layer.
Before You Begin
Leeks come in a variety of sizes. If possible, purchase ones that have more white than green. If you can find only predominately green leeks, then buy closer to 5 pounds of leeks to ensure that you have enough to yield the 10 cups of sliced leeks needed for this recipe. To make vegetarian: You can substitute vegetable broth for the chicken broth, if desired.
Instructions
- Adjust oven rack to middle position and heat oven to 450 degrees. Melt 1 tablespoon butter in 12-inch nonstick skillet over medium heat. Add panko and ¼ teaspoon salt and cook, stirring frequently, until golden brown, about 5 minutes. Transfer to bowl; set aside. Wipe skillet clean with paper towels.
- Melt 2 tablespoons butter in now-empty skillet over medium heat. Add leeks (skillet may be full), water, and ½ teaspoon salt. Cover and cook until leeks are tender, 7 to 9 minutes. Uncover and continue to cook until water has evaporated, about 2 minutes longer. Transfer leeks to 13 by 9-inch baking dish and spread in even layer; set aside.
- Melt remaining 2 tablespoons butter in now-empty skillet over medium-high heat. Add cremini mushrooms, shiitake mushrooms, pepper, and remaining ¾ teaspoon salt. Cover and cook until mushrooms have released their moisture, about 4 minutes. Uncover and continue to cook, stirring occasionally, until mushrooms are deep golden brown and tender, 6 to 8 minutes longer.
- Stir in flour, garlic, and thyme and cook for 1 minute. Stir in cream and broth and bring to boil. Off heat, stir in Parmesan until smooth. Pour mushroom mixture over leeks in baking dish.
- Transfer to oven and bake until bubbling around edges, 5 to 7 minutes. Transfer dish to wire rack and let cool for 10 minutes. Sprinkle with toasted panko. Serve.TO MAKE AHEAD: At end of step 4, let casserole cool completely. Cover dish with aluminum foil and refrigerate for up to 24 hours. Store panko in airtight container at room temperature. To serve, bake, covered, for 30 to 35 minutes.
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