1. In a medium Dutch oven or heavy pot, cook bacon over medium-high, stirring occasionally, until browned, about 5 minutes. Pour off all but 1 tablespoon fat from pot (reserve for another use). Add onions, and reduce heat to medium. Cook, stirring occasionally and scraping up browned bits from pot with a wooden spoon, until onions are very soft and deep golden brown, about 1 hour (reduce heat if onions are overbrowning). Add chicken broth and bring to a boil. Remove from heat.

2. Heat broiler, with rack in top position. With a ladle, transfer soup into six 8-ounce broilerproof baking dishes set on a rimmed baking sheet. Top each with a baguette slice and 1/4 cup grated cheese. Transfer sheet to oven and broil until cheese is melted and browned in spots, about 3 minutes. Serve immediately.