Preparation 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. 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. Preheat broiler. 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. Cooks' note:Creamed onions, without cheese, can be made (but not gratinéed) 1 day ahead. Chill, uncovered, until cool, then covered. Bring to room temperature, then reheat in a 400°F oven about 20 minutes before sprinkling with cheese and broiling.
Preparation 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. 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. Preheat broiler. 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. Cooks' note:Creamed onions, without cheese, can be made (but not gratinéed) 1 day ahead. Chill, uncovered, until cool, then covered. Bring to room temperature, then reheat in a 400°F oven about 20 minutes before sprinkling with cheese and broiling.