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.