Vegetarian French Onion Soup

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.

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.

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

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

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

Arrange bread slices on a baking sheet.

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

Carefully remove cheese rind from the pot and discard.

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.

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