Melt the butter in a saucepan over medium heat. Add the onions and a pinch of salt. Stir well. Cook for about 15 minutes, until the onions are lightly caramelized. If necessary, turn down the heat to keep the onions from burning. Stir frequently.

Preheat a broiler.

Add the flour, stir to combine, and continue cooking for another 5 minutes.

Slowly incorporate the broth, stirring continually. Increase the heat, bring to a boil, reduce heat, and simmer for another 10 minutes.

In the meantime, toast the baguette slices under the broiler.

Divide the soup between serving bowls. Arrange a layer of baguette slices on top of the soup. Sprinkle the grated cheese evenly over the baguette slices. Place the bowls under the broiler for about a minute to melt the cheese. Leave it slightly longer if you want the cheese to brown a bit.