I know that kale is the original recipe, but I've always liked spinach better. I think it has a nice sour kick to it. The problem is that spinach gets soggy much faster and quickly loses its texture. Is there any way to make the spinach stay firm longer in the soup?
It's common to add the spinach to a soup like this by putting the raw spinach in a bowl and ladling the hot soup over it. This protects the spinach from overcooking and losing both flavor and color before being served. Once the spinach is actually in the soup, there isn't much that you can do to keep it from getting soggy.