In a large soup pot, melt butter over medium-low heat.
Once butter is foamy, add sliced onions and sugar. Cook stirring occasionally until the onions are golden brown, this should take about 45 minutes - don't rush it.
Once cooked add in flour and stir to coat, cook for 3-4 minutes, stirring occasionally.
Add in vegetable broth, herbs, salt, and pepper to taste, then bring to a boil - turn the heat down to low and simmer for one hour.
While the soup is simmering, slice bread thin and toast in a 450-degree oven until lightly browned, 5-7 minutes.
Once the soup has cooked, ladle into 4 ovenproof dishes.
Top with the toasted french bread, cover each crock with a slice of provolone, and top that with the grated gruyere.
Place under the broiler and cook until the cheese is bubbly and lightly browned.