Bistro French Onion Soup
This cozy Bistro French Onion Soup is rich and deeply flavorful, made with slow-caramelized onions and a buttery broth. It’s topped with toasted croissant croutons and melted Swiss cheese for a warm, bistro-style meal that feels simple yet special.
Ingredients
- 4 tbsp butter
- 3 medium-sized onions (thinly slices)
- 1 tbps sugar
- 1 tbsp flour (optional, for a thicker broth)
- 4 cups beef broth
- 2 tbsp beef bouillon (optional)
- 1 tbsp balsamic vinegar or dry sherry (optional)
- 1 tsp fresh thyme (or ½ tsp dried thyme)
- salt & pepper (to taste)
- 2 croissants (sliced or torn into pieces)
- 1 cup Swiss or Gruyére cheese (shredded)
Instructions
Fat
- Melt butter in a large Dutch oven or soup pot over medium heat.
Flavor Base
- Add sliced onions, sugar, and salt. Cook slowly, stirring often, for 25–30 minutes, until deep golden brown and caramelized. If the onions start to stick, add a splash of water and scrape up the browned bits.
Liquid Base
- Sprinkle flour over the onions (if using) and stir for 1 minute. Add balsamic vinegar or sherry, then pour in the broth and add the bouillon. Add thyme and pepper. Simmer uncovered for 20 minutes to develop rich flavor.
Protein
- Optional – this soup is delicious as-is, with the onions as the main star.
Mix-ins
- Optional – the onions from the base and the crouton and cheese finishes add body.
Spices
- Taste and adjust seasoning with salt, black pepper, and thyme as needed.
Finishes
- Slice or tear croissants into pieces. Arrange on a baking sheet, top with shredded Swiss, and toast at 400°F for 5–7 minutes until golden and crisp. Ladle soup into bowls and top with cheesy croissant croutons. Garnish with fresh thyme before serving.
Notes
- For a deeper flavor, add a splash of balsamic vinegar or dry sherry while the soup simmers.
- If you love rosemary, a very small sprig or a few leaves can be beautiful here. Just use it lightly, as rosemary can quickly take over the flavor.
- Croissants make a lovely stand-in for traditional baguette croutons — buttery, crisp, and perfect for soaking up the rich broth. Any kind of bread could be used here though.