1. Turn the crockpot on to the sauté setting. Melt the butter with the olive oil. Add the sliced onions. Sprinkle in salt and sugar and cook, stirring occasionally, until deeply caramelised, about 20-25 minutes.
2. Add your flour to the onion mixture, then stir to coat everything. Next, cook for another minute.
3. Pour in the white wine to deglaze, scraping up any browned bits on the bottom, and reduce by half, then add the beef broth, Worcestershire, bay leaf and thyme.
4. Cover and cook on low for 6-8 hours or high for 3-4 hours.
5. Before serving, remove the bay leaf and thyme stems, then season to taste with salt and pepper.
6. To serve, ladle the soup into heatproof bowls, top with the baguette slices, sprinkle with the Gruyère and heat under the broiler until bubbly and golden.
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