In a large heavy-bottomed pot or Dutch oven, melt 3 tbs unsalted butter with 1 tbs olive oil over medium heat. Add the sliced onions, 1 tsp sea salt, and 1 tsp granulated sugar. Toss to coat. Cook, stirring every 3–5 minutes, for 30–40 minutes until onions are deeply golden-brown and caramelized. If they begin to stick, reduce heat slightly and scrape browned bits from the bottom with a wooden spoon.

Add flour

Push the onions to the sides and sprinkle 2 tbs all-purpose flour over the onions. Cook for 1–2 minutes, stirring constantly, to remove raw flour taste and to lightly toast; this will help thicken the soup slightly.

Deglaze

Pour in 120 ml dry white wine (or skip if not using) and stir, scraping any browned fond from the bottom. Allow the wine to reduce by half, about 2–3 minutes.

Add stocks and aromatics

Add 1 L beef stock and 500 ml chicken stock to the pot, scraping to incorporate any stuck bits. Add 3 sprigs fresh thyme and 2 bay leaves. Bring the soup to a gentle simmer, then reduce heat to low and simmer, uncovered, for 25–30 minutes to meld flavors.

Finish soup

If using, stir in 1 tbs sherry or brandy off the heat. Taste and adjust seasoning with additional salt and freshly ground black pepper as needed (approximately another 1/2 tsp salt and up to 1/2 tsp pepper depending on your stock). Remove thyme sprigs and bay leaves before serving.

Topping / croûtes

Prepare bread slices

Preheat broiler (grill) or oven to high (225–250°C / 435–480°F). Brush both sides of the 8 bread slices lightly with 1 tbs olive oil or melted butter. If using, brush one side with 1 tsp Dijon mustard for extra flavor.

Toast bread

Place the bread slices on a baking sheet and toast under the broiler or in the oven for 2–3 minutes per side, or until golden and crisp. Remove and set aside.

Assemble croûtes

Mix 300 g grated gruyère with 50 g grated parmesan. Pile a generous handful of the cheese mixture on each toasted bread slice so the cheese mound is thick enough to melt and brown.

Broil croûtes

Place the cheese-topped bread back under the broiler until the cheese is fully melted, bubbling, and lightly browned on top, about 2–4 minutes. Watch closely to prevent burning.

To serve

Ladle the hot soup into ovenproof bowls or crocks, about 350–400 ml soup per bowl, leaving a little room at the top for the croûte.

Float one cheese-topped croûte on each bowl of soup, or place the croûte on top of the soup and, if desired, place the bowls on a baking sheet and put under the broiler for 1–2 minutes to brown edges and meld cheese with soup. Be careful when handling hot bowls.

Garnish with 1 tbs chopped fresh parsley and serve immediately.