This will be your new favorite soup recipe. Hands. Down.
- Add butter, olive oil, and onions to a large skillet over medium heat. Cook until onions start to caramelize.
- Pour onions into the slow cooker and top with beef broth, bay leaf, salt, and pepper.
- In a small bowl whisk together flour, balsamic vinegar, and Worcestershire sauce until smooth. Stir into soup.
- Cover and cook on low for 6 to 8 hours, or on high for 3 to 4 hours.
- Scoop soup into oven-safe bowls. Top with 1 slice of toasted bread and cheese. Broil for 5 minutes, or until cheese is melted and bubbly.
- Top with chives, serve and enjoy!