Prepare Onions

1. Peel and thinly slice all onions into even 3-4 mm slices.
2. In a large heavy-bottomed pot, melt butter with olive oil over medium heat.
3. Add sliced onions, stir to coat, then cook, stirring every few minutes, until softened (10–15 min).
4. Reduce heat to medium-low, add sugar and salt, and continue to cook, stirring frequently, until onions are deep golden brown and caramelized (30–40 min).

Finish Soup

1. Increase heat to medium-high and add the white wine to deglaze, scraping up browned bits; simmer until reduced by half (2–3 min).
2. Stir in beef stock, thyme sprigs, bay leaves and cognac if using; bring to a gentle simmer.
3. Lower heat and simmer uncovered 20–25 minutes to blend flavors; skim any foam and adjust seasoning with salt and pepper.
4. Remove thyme sprigs and bay leaves before serving.

Bread & Broil

1. While soup simmers, preheat oven broiler on high and toast baguette slices until golden on both sides.
2. Ladle hot soup into ovenproof bowls, place 1–2 toasted baguette slices on each, sprinkle generously with grated Gruyère and a little Parmesan.
3. Place bowls on a baking sheet and broil 2–5 minutes, watching closely, until cheese is bubbly and browned.
4. Carefully remove from oven, let sit 1 minute, then serve hot.