Over medium heat, sauté onions in butter in the bottom of a stockpot until softened, 10-15 minutes. Sprinkle in flour, and stir to coat evenly.
Gradually add chicken stock. Bring to a boil. Cover, and reduce heat to simmer. Simmer 40 minutes.
Meanwhile, toast baguette toast rounds in a toaster oven. (To use your regular oven, turn heat to very low and keep an eye on them until they are golden brown.)
Divide soup into oven safe bowls and float a piece of the toast in each bowl. Sprinkle with just enough grated cheese to hide the bread.
Broil until cheese is golden.
Adapted from
The French Don't Diet Plan