3 Onions Soup Recipe

Prepare onions

In a large heavy-bottomed pot or Dutch oven, heat butter and 1 tbsp olive oil over medium heat until butter foams. Add the sliced onions and sprinkle with 1 tsp sea salt and 1 tsp sugar. Cook, stirring frequently, until onions become very soft and translucent, about 15 minutes.

Caramelize onions

Turn heat to medium-low and continue to cook the onions, stirring every few minutes to prevent burning, until they are deeply golden-brown and richly caramelized, about 30–40 minutes. If the pan looks dry at any point, add 1–2 tablespoons water to deglaze. Adjust heat to maintain a gentle sizzle, not a rapid fry.

Build the soup base

Raise heat to medium and sprinkle the 1 tbsp flour over the onions; stir and cook 1–2 minutes to remove raw flour taste. Carefully add the white wine (120 ml) to deglaze, scraping up browned bits from the bottom. Let the wine reduce by half, about 2–3 minutes.

Add stocks and aromatics

Add the beef stock (1 L) and chicken stock (500 ml) to the pot. Add the thyme sprigs and bay leaves. Stir in 1 tsp Worcestershire sauce and 1 tsp sherry vinegar. Bring soup to a gentle simmer, then reduce heat to low and simmer, partially covered, for 20–30 minutes to meld flavors.

Finish soup

Taste and remove thyme sprigs and bay leaves. Adjust seasoning with additional salt and freshly ground black pepper to taste. If soup tastes flat, add an extra 1/2 tsp Worcestershire or a splash of vinegar, then simmer 2 minutes more. Keep soup hot while preparing crostini.

Toast baguette slices

Preheat oven to 200°C (400°F). Arrange baguette slices in a single layer on a baking sheet. Brush each slice lightly with olive oil (about 2 tbs total). Toast in the oven until golden and crisp, 6–8 minutes, flipping once halfway through.

Rub with garlic

Remove to a rack and immediately rub the cut side of the halved garlic clove over the warm toasts for a subtle garlic flavor.

Assemble cheese crostini

Divide grated Gruyère (1 cup) and shredded provolone (1 cup) evenly over the toasted slices. Return to the oven (or under a broiler) and melt until cheese is bubbly and lightly browned, 2–4 minutes. Watch closely to avoid burning.

Optional finish

If desired, sprinkle a little finely grated Parmesan on each crostini as soon as they come out of the oven.

Portion soup

Ladle the hot onion soup into ovenproof bowls or crocks, filling about 3/4 full to allow room for the crostini.

Top with crostini

Place one or two cheese-topped crostini on each bowl, or float the crostini on top of the soup. If you prefer the classic gratinéed presentation, place bowls on a baking sheet, top each with grated Gruyère and provolone, and broil until cheese is bubbly and golden—about 2–4 minutes (watch carefully).

Garnish and serve

Sprinkle chopped parsley and a little extra black pepper or optional Parmesan over each bowl. Serve immediately while hot.