Stout Onion Soup
In large saucepan, melt butter over medium heat; cook onions, stirring often, until softened, about 10 minutes. Add maple syrup and vinegar; cook, stirring occasionally, until onions are tender and golden, about 10 minutes. Season with salt and pepper. Add garlic and thyme; cook, stirring, for 30 seconds.
Sprinkle flour over onions; mix to coat well. Add broth, beer and mustard, scraping up browned bits from bottom of pan. Bring to boil; reduce heat and simmer for 10 minutes. (Make-ahead: Can be refrigerated for up to 3 days or stored in airtight container and frozen for up to 3 months.)
Preheat broiler. Divide onion soup among ovenproof bowls. Arrange 2 slices of bread in each bowl; top with cheese. Sprinkle with pepper. Bake until cheese is melted and golden, about 2 minutes.