French Onion Recipe

Prepare onions

Peel and halve the onions from root to tip. Lay the flat side down and slice into thin (about 1/8") half-moon slices. Aim for consistent thickness so they caramelize evenly.

Cook onions low and slow

In a large heavy-bottomed pot or Dutch oven, heat the butter and olive oil over medium heat until butter foams. Add the sliced onions and sprinkle with the Kosher salt and granulated sugar. Stir to coat. Reduce heat to medium-low. Cook, stirring every 3–5 minutes, scraping browned bits from the bottom, until onions are deeply golden brown and jam-like, about 35–45 minutes. If they start to stick excessively or brown too fast, lower the heat.

Deglaze

Once the onions are richly browned, push them to the side and add the tablespoon of flour; cook for 1 minute to remove raw taste, stirring to incorporate into the onions. Pour in the dry white wine (or sherry) to deglaze the pan, scraping up any fond from the bottom. Let the wine reduce by about half, about 2–3 minutes.

Simmer soup

Add the beef stock, bay leaf, and thyme sprigs. Bring to a gentle simmer, then reduce heat to low and simmer uncovered for 20 minutes to meld flavors. Stir in Worcestershire sauce, freshly ground black pepper, and taste for seasoning; add more salt if needed. Remove and discard the bay leaf and thyme stems. If using, stir in the lemon juice to brighten the flavor.

Toast baguette slices

While the soup simmers, preheat the oven broiler. Arrange baguette slices on a baking sheet, brush lightly with olive oil or rub with a cut garlic clove if desired, and broil until crisp and golden on both sides, about 1–2 minutes per side. Alternatively, toast in a toaster oven.

Assemble and brown

Preheat oven to 450°F (230°C) with rack in the top third, or use a broiler-safe ovenproof bowl. Ladle the hot soup into ovenproof bowls or crocks. Place 1–2 toasted baguette slices on top of each bowl and pile a generous handful of grated Gruyère (and a sprinkle of Parmesan if using) over the bread. Place bowls on a baking sheet and bake at 450°F for 6–8 minutes until cheese is melted and bubbly, then switch to broil for 1–2 minutes to achieve a golden-brown crust. Watch closely to avoid burning.

Finish and serve

Carefully remove hot bowls from the oven (use oven mitts). Let rest 2 minutes, garnish with a small thyme leaf if desired, and serve immediately. Provide soup spoons and warn diners the bowls are hot.