In a large heavy-bottomed pot, heat butter and olive oil over medium heat. Add sliced onions, ½ tsp salt, and sugar. Cook slowly, stirring occasionally, until onions are golden and deeply caramelized (40–45 mins).

Add minced garlic and cook for 1–2 minutes. Sprinkle flour over onions, stir to coat, and cook for 2 minutes to remove raw flour taste.

Deglaze the pan with white wine (if using), scraping the bottom of the pot to lift caramelized bits. Reduce for 2–3 minutes.

Add beef broth, thyme sprigs, and bay leaf. Bring to a gentle boil, then reduce heat and simmer for 20–25 minutes. Remove thyme and bay leaf before serving.

Preheat oven broiler. Arrange toasted baguette slices on a baking sheet, top with grated Gruyère, and broil until cheese is bubbly and golden (2–3 minutes). Watch carefully.

Ladle hot soup into oven-safe bowls. Place one or two cheesy baguette slices on top. Optionally, broil assembled bowls for 1–2 minutes for extra bubbly topping. Garnish with fresh parsley and serve immediately.