1Melt butter in a large saucepan over low heat. Add onions; cook, stirring constantly, until golden, 15 to 30 minutes. Add water; cover and cook until onions are very tender, about 15 minutes.
2Sprinkle flour into the saucepan; cook, stirring constantly, until onions are coated, about 5 minutes. Pour in chicken stock and 1/3 cup plus 1 tablespoon white wine; season with salt and pepper. Bring to a boil; reduce heat and simmer gently until flavors combine, about 20 minutes.
3Set oven rack about 6 inches from the heat source and preheat the oven's broiler.
4Toast baguette slices until very crisp, 3 to 5 minutes. Divide slices among 6 oven-safe soup bowls. Ladle soup on top. Sprinkle Gruyère cheese over each bowl.
5Transfer bowls to the preheated oven; broil until cheese is melted and bubbly, 3 to 5 minutes.
Footnotes
Cook's Notes:
Do as the French do and remove the inner core of the onions before slicing. The French believe this makes the onion easier to digest.
Substitute Comté cheese for the Gruyere if desired.