Vegetarian French Onion Soup

Step 1
Melt butter and olive oil in a pot over medium heat. Add red onions and sweet onions; sprinkle with kosher salt and sugar. Cook, stirring frequently, until onions are very tender and caramelized, about 45 minutes.

Step 2
Pour mushroom broth, sherry, Worcestershire sauce, parsley, herbes de Provence, and bay leaves into onion mixture. Bring to a simmer and cook over medium-low heat for 45 to 60 minutes.

Step 3
Reduce heat to low and remove bay leaves. Pour balsamic vinegar into soup and adjust seasoning. Mix well.

Step 4
Place Parmesan cheese rind in the center of the pot and submerge, but do not stir. Cook soup for an additional 20 minutes.

Step 5
Set an oven rack about 6 inches from the heat source and preheat the oven's broiler.

Step 6
Arrange bread slices on a baking sheet.

Step 7
Broil bread slices in the preheated oven for 2 to 3 minutes per side. Remove from the oven and set aside.

Step 8
Carefully remove cheese rind from the pot and discard.

Step 9
Fill 4 oven-safe bowls evenly with soup and sprinkle each with a light layer of grated Gruyere cheese and Parmesan cheese. Top each with 2 bread slices and sprinkle each with remaining Gruyere cheese and Parmesan cheese. Place bowls on a baking sheet.

Step 10
Broil soup in the preheated oven until cheese is browned and bubbling, 3 to 5 minutes.