Vegetarian French Onion Soup

2 teaspoons unsalted butter
1 tablespoon vegetable oil
5 large onions, halved and thinly sliced
3 garlic cloves, minced
1/2 teaspoon salt
2 tablespoons all-purpose flour
6 cups roasted vegetable broth
1/3 cup dry sherry (optional)
1 teaspoon Dijon mustard
1 teaspoon sherry wine vinegar (Substitute: use ¼ tsp lime juice)
6 slices sourdough bread, toasted (1/2 inch thick) (Substitute: any regular bread will do.. just toast it a little so it does not become squishy
2/3 cup cheese, shredded

Directions:

1 Heat butter and oil in heavy, large saucepan over medium heat. Add onions, garlic and salt, and cook 5 minutes, stirring often. Reduce heat to very low, and cook, stirring occasionally, 50 to 60 minutes, until onions are very tender and brown.
2 Stir in flour, and cook 3 to 4 minutes, stirring constantly. Blend in broth, sherry and mustard. Cover, and simmer 15 minutes, stirring occasionally. Stir in sherry vinegar.
3 Preheat broiler, ladle soup into six heat-proof bowls. Top each with slice of toast and grated cheese. Broil until cheese is bubbly and golden, about 1 to 2 minutes. Serve hot.