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1
Preheat the oven to 350 degrees Fahrenheit. Spread the bread cubes on a sheet pan and bake for 15 to 20 minutes, until lightly browned. Set aside.
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2
Cut the leeks lengthwise, slice crosswise 1/4 inch thick, wash in a big bowl of water, and spin-dry in a salad spinner. Don't put mushrooms in water; wipe them with a clean, damp sponge.
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3
Meanwhile, heat the oil and butter in a large (12-inch) saute pan over medium heat. Add the pancetta and cook for 5 minutes, until starting to brown. Stir in the leeks and cook over medium heat for 8 to 10 minutes, until the leeks are tender. Stir in the mushrooms, tarragon, sherry, 1.
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4
tablespoon salt and 1 teaspoon pepper, and cook for 10 to 12 minutes, until most of the liquid evaporates, stirring occasionally. Off the heat, stir in the parsley.
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5
In a large mixing bowl, whisk together the eggs, cream, chicken stock and 1 cup of the Gruyere. 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. Stir well and pour into a 2 1/2- to 3-quart gratin dish (13x9x2 inches). Sprinkle with the remaining 1/2 cup of Gruyere and bake for 45 to 50 minutes, until the top is browned and the custard is set. Serve hot.
