Vegetarian French Onion Soup

1 . Stage

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.

2 . Stage

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.

3 . Stage

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

4 . Stage

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

5 . Stage

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

6 . Stage

Arrange bread slices on a baking sheet.

7 . Stage

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

8 . Stage

Carefully remove cheese rind from the pot and discard.

9 . Stage

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.

10 . Stage

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