Ingredients

• 2 pound white pearl onions
• 2 tablespoons unsalted butter
• 2 tablespoons all-purpose flour
• 1 cup whole milk
• 3 tablespoons cream Sherry
• 1 tablespoon grainy mustard
• 1 tablespoon Dijon mustard
• 1/4 teaspoon grated nutmeg
• 1/2 cup grated Parmigiano-Reggiano
• Equipment: a 2-qt shallow gratin or other flameproof baking dish (2 inches deep)

Instructions

1. Cook onions in a large heavy pot of boiling salted water (2 teaspoons salt for 4 quarts water) until just tender, 25 to 30 minutes. Reserve 1 cup cooking water, then drain (reserve pot). Cool onions to warm, then peel.
2. Melt butter in dry pot over medium heat. Add flour and cook, stirring, 2 minutes. Whisk in reserved cooking water, milk, and Sherry and cook, whisking frequently, until mixture is thickened, about 8 minutes. Whisk in mustards, nutmeg, and 1/2 teaspoon each of salt and pepper. Add onions to sauce and simmer 5 minutes.
3. Preheat broiler.
4. Transfer creamed onions to baking dish and sprinkle evenly with cheese. Broil 4 to 5 inches from heat until top is golden brown and bubbling, about 3 minutes.