3 large heads garlic, left whole, plus 3 garlic cloves, smashed
1 tablespoon olive oil
2 1/2 cups whole milk
1 medium onion, sliced
3 large thyme sprigs plus 1 teaspoon thyme leaves
1 bay leaf
1 whole clove
1/8 teaspoon black peppercorns
7 tablespoons unsalted butter, divided
1 1/2 tablespoons fine dry bread crumbs
5 tablespoons all-purpose flour
4 large eggs, separated, plus 4 additional egg whites
1 1/2 cups grated Parmigiano-Reggiano (3 ounces)
1/4 teaspoon grated nutmeg
3/4 cup grated Gruyère (2 ounces)
Preheat oven to 400°F with rack in middle.
Trim 1/4 inch from tops of whole heads of garlic, then put heads on a large sheet of foil and drizzle each with 1 teaspoon oil. Wrap heads together in foil and roast until very tender, 50 minutes to 1 hour. Cool to warm, then squeeze garlic from skins.
Meanwhile, bring milk, smashed garlic cloves, onion, thyme sprigs, bay leaf, clove, and peppercorns just to a boil in a medium saucepan, then remove from heat and cover. Let steep 30 minutes.
Butter a 12-inch oval (2-quart) gratin dish with 1 tablespoon butter, then sprinkle bottom and side with bread crumbs and chill until ready to use.
Melt remaining 6 tablespoons butter in a heavy medium saucepan and whisk in flour. Cook roux over low heat, whisking constantly, 5 minutes.
Strain milk mixture through a fine-mesh sieve, then add to roux, whisking until smooth. Bring to a boil, whisking, then simmer béchamel, whisking, 3 minutes. Remove from heat and vigorously whisk in yolks 1 at a time. Whisk in roasted garlic, parmesan, nutmeg, 3/4 teaspoon salt, and 1/4 teaspoon pepper and transfer to a bowl.
Beat egg whites with a pinch of salt using an electric mixer until they just hold stiff peaks, then stir one third of egg whites into yolk mixture. Fold in Gruyère, then remaining whites. Transfer mixture to gratin dish, smoothing top, and sprinkle with thyme leaves.
Bake soufflé until set and browned on top, 20 to 25 minutes.
Recipe from Epicurious.com