Thinly slice the onions- 5-6 cups. In a large soup saucepan, over medium heat, melt the butter and add the onion. Cook until the onion is limp and soft- about 20 minutes, stirring often. (this makes the onion nice and sweet)
Turn up the heat to medium high and cook the onion until golden brown, but do not burn. It takes 10 minutes. Turn the heat back down to medium.
Add 2 tablespoons all purpose flour and cook for 2 minutes.
Add broth, water, salt, 2 tablespoons wine, mustard and 1 bay leaf. Bring to a boil, stirring frequently.
Turn the heat down to a high simmer and cook with the lid off for 30 more minutes. Taste and season with more salt and pepper if needed.