Ingredients
1 1/4 cups whole milk
3/4 cup heavy cream
1 small celery root (about 12 ounces), peeled and cut into 1/2-inch pieces (about 2 cups)
Coarse salt and freshly ground pepper
1/2 loaf sourdough bread (about 8 ounces), cut into 1-inch cubes (about 7 cups)
2 tablespoons unsalted butter, plus more, room temperature, for baking dish
1 pound mixed wild mushrooms, such as shiitake, maitake, cremini, and oyster, trimmed and cut into 1/4-inch slices (about 7 cups)
2 large eggs, lightly beaten
1 tablespoon fresh thyme leaves, plus sprigs for garnish
1/2 cup crumbled mild blue cheese (about 2 ounces), such as Bayley Hazen Blue
Preparation
Preheat oven to 400 degrees. Combine milk, cream, and celery root in a small saucepan. Bring to a boil, then reduce heat and simmer until celery root is fork-tender, about 10 minutes. Let cool slightly. Transfer mixture to a blender; puree until smooth. Season with salt and pepper.
Meanwhile, lightly toast bread on a rimmed baking sheet, tossing once, until golden and crisp, about 10 minutes.
In a large skillet, melt butter over medium-high heat. Add mushrooms and cook, stirring occasionally, until they have released most of their moisture and become golden, about 9 minutes. Add 2 tablespoons water and bring to a boil, scraping up browned bits with a wooden spoon; cook until water evaporates. Season with salt and pepper. Transfer to a bowl.
Butter a shallow 2-quart baking dish. In a large bowl, whisk together celery-root mixture, eggs, and thyme leaves. Fold in toasted bread and mushrooms. Transfer to prepared dish; sprinkle with cheese. Garnish with thyme sprigs and bake until custard is set and top is lightly browned, 20 to 25 minutes. Let cool slightly before serving.
