If you haven’t tried the fall menu at Siena Restaurant at 12th and Granville, you are missing out. The mushroom-themed $24 is on through November. Crimini & Oyster Mushroom Soup  crispy garlic chips, toasted pine nuts or Chanterelles on Brioche  madeira thyme cream, aged balsamico, arugula Grilled Lamb Sirloin & Morels  red wine jus, rosemary and…