4 cups vegetable broth
2 1/2 pounds medium onions, halved and thinly sliced lengthwise
2 bay leaves
1/2 teaspoon fine sea salt
4 tablespoons unsalted butter
1/4 cup plus 1 tablespoon unbleached all-purpose flour
Scant 1/2 teaspoon coarsely crumbled saffron threads
1/2 baguette, cut into 1-inch cubes
6 ounces Toma or Gruyère cheese, thinly sliced with a cheese plane (see note)
SPECIAL EQUIPMENT: 4 ovenproof individual soup crocks
In a medium saucepan, combine broth and 2 1/2 cups water; bring just to a boil, then remove from heat and cover to keep warm.
Meanwhile, in a 4- to 5-quart heavy pot, cook onions, bay leaves and salt in butter over medium heat, uncovered, stirring frequently, 10 minutes, then cover and cook 10 minutes more. In 3 to 4 additions, sprinkle flour over onions, stirring to combine as you go. Cook
1 minute, then add hot broth mixture. Return liquid to a simmer, then reduce to a very gentle simmer and cook, partially covered, 45 minutes. Stir in saffron; continue to gently simmer, uncovered, 1 hour more.
Meanwhile, heat oven to 350° with rack in middle. Arrange bread cubes in 1 layer on a large baking sheet and toast until dry and lightly golden, about 15 minutes. Remove croutons from oven.
When soup is ready, heat broiler. Put heatproof crocks on a rimmed baking sheet; add 2 croutons to each. Remove and discard bay leaves from soup; divide soup among crocks, then float remaining croutons on top. Top with cheese, allowing ends of cheese to hang over rims of crocks. Broil 4 to 5 inches from heat until cheese is melted and bubbly,
1 to 2 minutes.
Note: Toma is a cow’s milk cheese produced in Piedmont and Lombardy, similar to, though a bit sweeter than, Gruyère. Look for it in Italian markets or cheese shops. Soup and croutons can be made 3 days ahead (but do not add croutons and cheese to soup); cool completely, uncovered, then chill soup, covered, and keep croutons in an airtight container at room temperature. Reheat soup before proceeding with recipe.