Onion Soup Recipe

Prepare onions

Peel and halve the onions, then thinly slice each half into uniform 2–3 mm slices. Separating any larger layers so they cook evenly.

Caramelize onions

In a large heavy-bottomed pot or Dutch oven, heat 4 tbs unsalted butter and 1 tbs olive oil over medium heat until butter melts and begins to foam. Add the sliced onions, 1 tsp salt and 1 tsp sugar. Toss to coat. Cook, stirring frequently, for 30–45 minutes, reducing heat to medium-low if they start to brown too quickly. Scrape the bottom occasionally to deglaze and ensure even caramelization. The onions should become deeply golden-brown and very soft.

Add flour and deglaze

Increase heat to medium, sprinkle 1 tbs all-purpose flour over the onions and cook for 1–2 minutes while stirring to remove raw flour taste and lightly toast the flour. Pour in 120 ml dry white wine or sherry to deglaze the pot, scraping any brown bits from the bottom. Cook until the wine has mostly evaporated, about 2–3 minutes.

Simmer with stock

Add 1.2 l beef stock, 2 bay leaves, 3 thyme sprigs and 1 tsp Worcestershire sauce. Bring to a gentle simmer, then reduce heat and simmer uncovered for 20–25 minutes to let flavors meld. Taste and adjust seasoning with additional salt and 1 tsp freshly ground black pepper if needed. If desired, stir in 1 tsp sherry vinegar or a squeeze of lemon to brighten the broth just before serving.

Finish and strain (optional)

Remove and discard bay leaves and thyme sprigs. For a smoother broth, ladle through a fine-mesh sieve into a clean pot, pressing on solids to extract flavor. Keep warm at low heat while preparing bread and cheese.

Toast the bread

Preheat oven to 200°C (400°F) or set broiler on high. Brush both sides of the 6 bread slices lightly with 1 tbs olive oil (or spread softened butter). Place slices on a baking sheet and toast in the oven until golden and crisp, about 6–8 minutes, flipping once. Under the broiler, watch closely: 1–2 minutes per side may be enough.

Assemble with cheese

Ladle hot soup into oven-safe bowls or crocks, leaving room at the top for the bread and cheese. Place one toasted bread slice on top of each bowl of soup. Sprinkle about 50 g grated Swiss cheese over each slice (adjust to preference). Optionally add a light sprinkle of grated Parmesan on top for extra umami.

Broil to melt cheese

Place filled bowls on a baking sheet and put under the preheated broiler (or in a very hot oven) until the cheese is bubbling and golden-brown, about 3–5 minutes. If your bowls are not oven-safe, use a salamander or transfer to a baking dish briefly, or melt the cheese on the bread separately and place on top of the soup just before serving.

Remove bowls from the oven carefully using oven mitts. Sprinkle with 1 tsp chopped fresh thyme leaves. Let rest 1–2 minutes to cool slightly, then serve hot. Provide extra ground black pepper at the table if desired.