These are fancier than your everyday, garden-variety open-face sandwiches. It all starts with a bed of golden (but not quite caramelized) onions, a dollop of sour cream, crisp asparagus and crumbled feta. Dashes and splashes of sumac, parsley and lemon juice brighten up the party considerably. For textural oomph and depth of flavour, make sure to use good country-style bread.