In a medium sized pan, heat oil, add sweet onion, brown sugar and sherry, and cook over medium high heat, stirring frequently until browned, for about 20 minutes. Set aside.
Add in water, stock, dijon, miso paste, sherry, salt and pepper, bring to a boil and reduce to a simmer.
Meanwhile, combine ingredients for tapenade in a bowl and set aside.
Pour soup into oven safe bowls, grate cheese on top, put 2-3 crostinis on top of each bowl, add a spoonful of tapenade to cover crostinis, sprinkle a little more cheese on top, and put under broiler for 1 minute.