Loosely adapted from Ina Garten in Barefoot Contessa Foolproof: Recipes You Can Trust
Try this for your gluten-free and vegetarian friends. Rich, flavorful and full of texture.
Preheat the oven to 350°F. Spread the bread cubes on a sheet pan and bake for 15 to 20 minutes, until lightly browned. Set aside.
Meanwhile, heat the oil and butter in a large (12-inch) sauté pan over medium heat. Add the leeks and cook over medium heat for 8 to 10 minutes, until the leeks are tender. Stir in the mushrooms, tarragon, sherry, salt, and pepper and cook for 10 to 12 minutes, until most of the liquid evaporates, stirring occasionally. Remove from the heat, stir in the parsley.
In a large mixing bowl, whisk together the eggs, cream, vegetable broth, and 1 cup of the shredded Gruyère. Add the bread cubes and mushroom mixture, stirring well to combine. Set aside at room temperature for 30 minutes to allow the bread to absorb the liquid.