How to Make It
Heat the olive oil in a medium skillet. Add the onion, season with salt and cook over moderate heat, stirring, until it begins to brown, about 8 minutes. Add the water and cook until it has evaporated and the onion is very tender and caramelized, about 10 minutes. Season with salt and let cool.
Preheat the oven to 350°. Arrange the Crostini Toasts on a baking sheet and top each with a slice of cheese. Bake for about 4 minutes, or just until the cheese melts. Top with the caramelized onion and serve.