- 2 sweet onions + 1 red onion, sliced into thin, short strips
- 1 tbsp minced garlic
- 4 tbsp butter, melted
- 2 tablespoons brown sugar
- 4-5 generous pinches of salt
- ¼ cup balsamic vinegar
- 2 tbsp Dijon mustard (optional)
- 3 tbsp all-purpose flour
- 56-64 ounces of beef stock or broth (4 cans or 2 boxes)
- Fresh ground black pepper to taste
- French bread
- Gruyere, Provolone, or Swiss cheese (sliced or grated)
Set your crock pot on high, then add onions, garlic, butter, sugar, vinegar, and salt, and mix until combined. Cover and let cook for 60 minutes, stirring occasionally, until onions are caramelized and turning brown on the edges. (If you’re in a hurry, do this on the stove top or in the microwave, then add to crock pot.) Add flour, then stir thoroughly and let sit for 5 minutes. Add beef stock and pepper, then turn heat down to low, cover and cook for 4-10 more hours.
Spoon soup into bowls. Top with a thick slice of French bread covered with cheese, toasted in oven until browned. Can slice bread into chunks or strips if desired.