Leek and Mushroom Gratin

INGREDIENTS

• 3/4 stick unsalted butter, divided
• 1 cup fine dry bread crumbs
• 1/4 pound Gruyère, finely grated on small teardrop-shaped holes of a box grater (2 cups)
• 1/2 cup finely grated Parmigiano-Reggiano
• 2 teaspoons finely chopped garlic
• 1 tablespoon finely chopped flat-leaf parsley
• 3 pounds leeks, root ends trimmed
• 2 tablespoons all-purpose flour
• 1 1/2 cups chicken stock or reduced-sodium chicken broth
• 1/8 teaspoon grated nutmeg
• 1/2 teaspoon grated lemon
• 1 pound cremini mushrooms, trimmed and thinly sliced
• Equipment: a 1 1/2-quart oval gratin or other shallow baking dish