1. For the soup: Combine onions and 1 teaspoon salt in large microwave-safe bowl and cover with large microwave-safe plate (plate should completely cover bowl and not rest on onions). Microwave on high power for 20 to 25 minutes until onions are soft and wilted, stirring halfway through cooking. (Use oven mitts to remove bowl from microwave and remove plate away from you to avoid steam burn.) Drain onions (about 1/2 cup liquid should drain off).

2. Place Dutch oven over medium-high heat and melt butter before adding wilted onions. Using oven mitts to handle pot, cook onions, stirring frequently and scraping bottom and sides of pot, until liquid evaporates and onions brown, 15 to 20 minutes, reducing heat to medium if onions are browning too quickly. Continue to cook, stirring frequently, until pot bottom is coated with dark crust, 6 to 8 minutes, adjusting heat as necessary. (Scrape any fond that collects on spoon back into onions.) Stir in 1/4 cup water, scraping pot bottom to loosen crust, and cook until water evaporates and pot bottom has formed another dark crust, 6 to 8 minutes. Repeat process of deglazing 2 or 3 more times, until onions are very dark brown. Stir in sherry and cook, stirring frequently, until sherry evaporates, about 5 minutes.

3. Stir in broths, 2 cups water, thyme, bay leaf, and 1/2 teaspoon salt, scraping up any final bits of browned crust on bottom and sides of pot. Increase heat to high and bring to simmer. Reduce heat to low, cover, and simmer 30 minutes. Remove and discard herbs, then season with salt and pepper.

4. For the croutons: While soup simmers, arrange baguette slices in single layer on baking sheet and bake in 400-degree oven until bread is dry, crisp, and golden at edges, about 10 minutes. Set aside.

5. To serve: Adjust oven rack 6 inches from broiler element and heat broiler. Set individual broiler-safe crocks on baking sheet and fill each with about 1 3/4 cups soup. Top each bowl with 1 or 2 baguette slices (do not overlap slices) and sprinkle evenly with Gruyère. Broil until cheese is melted and bubbly around edges, 3 to 5 minutes. Let cool 5 minutes before serving.

Serves 6