Mushroom and Leek Bread Pudding

Name: Mushroom and Leek Bread Pudding
Ingredients: 6 cups bread cubes (1/2-inch-diced), from a rustic country loaf, crusts removed; 2 tablespoons good olive oil; 1 tablespoon unsalted butter; 2 ounces pancetta, small diced; 4 cups sliced leeks, white and light green parts (4 leeks); 1 1/2 pounds cremini mushrooms, stems trimmed and 1/4-inch-sliced; 1 tablespoon chopped fresh tarragon leaves; 1/4 cup medium or dry sherry; Kosher salt and freshly ground black pepper; 1/3 cup minced fresh flat-leaf parsley; 4 extra-large eggs; 1 1/2 cups heavy cream; 1 cup chicken stock, preferably homemade; 1 1/2 cups grated Gruyere cheese (6 ounces)