Curly endive is bitter and a bit tough. For both those reasons, I love adding it to this soup. The bitterness in the greens is perfectly balanced with the saltiness of the broth and the heartiness of the meatballs. The texture gives it a nice chew. Curly endive is really one of the best vegetables you can add to this soup. If you need a substitution, kale is better than spinach because of the texture.