In a large Dutch oven, melt butter over medium heat. Add onions; cook, stirring occasionally, until softened, about 15 minutes. Add salt, pepper, and sugar; cooking, stirring occasionally, until onions are deep golden brown, 45 to 50 minutes.
Increase heat to high. Add wine; cook until most of liquid has evaporated. Stir in broth. Tie together thyme sprigs and bay leaves with kitchen twine; add to broth. Bring to a boil; reduce heat to medium, and cook, uncovered, until soup is slightly thickened, about 30 minutes.
Preheat oven to broil.
Place 6 (7-ounce) ramekins on a small baking sheet. Ladle soup into ramekins, filling each three-fourths full. Place 1 baguette slice on top of each serving. Sprinkle with cheese.
Broil until cheese is golden brown. Garnish with pepper, if desired.
Recipe by Southern Lady Magazine at https://southernladymagazine.com/s-french-onion-soup-recipe/