Prep:30min › Cook:5hr › Ready in:5hr30min
- 6 tablespoons butter
- 4 large Spanish onions, sliced and separated into rings
- 1 tablespoon sugar
- 2 cloves garlic, minced
- 120ml sherry
- 1.5L beef stock
- Salt, to taste
- 1/4 teaspoon dried thyme
- 1 bay leaf
- 8 slices of French baguette
- 60g Gruyere cheese, grated
- 40g Emmental cheese, grated
- 20g fresh Parmesan cheese, grated
- 2 tablespoons mozzarella cheese, grated
- Heat butter in a large, heavy pot over medium-high heat; cook and stir onions until they become soft, about 10 minutes. Sprinkle onions with sugar; reduce heat to medium. Cook, stirring constantly, until onions are soft and browned, at least 30 minutes. Stir in garlic and cook until fragrant, about 1 minute.
- Stir sherry into onion mixture and scrape bottom of pot to release any small bits of caramelised onion stuck to the bottom. Transfer onions into a slow cooker and pour in beef stock. Season to taste with sea salt; stir in thyme and bay leaf. Cover cooker, set on High, and cook 4 to 6 hours. If desired, set on Low and cook 8 to 10 hours.
- About 10 minutes before serving, arrange bread slices on a baking tray and toast both sides under the grill.
- Combine all the grated cheeses in a bowl, tossing lightly. Fill oven-safe soup bowls 3/4 full of onion soup and set a bread slice in each bowl. Top with about 2 tablespoons of cheese mixture per serving.
- Place bowls on a baking tray and grill until cheese topping is lightly browned and bubbling, about 2 minutes.
You don’t have caramelise the onions, you can simply melt the butter in your slow cooker, add the onions to coat them and then add all the ingredients and cook. However, caramelising the onions makes for a much, much better soup.