La Piazza Al Forno (Glendale, Ariz.) • Slim's Last Chance Chili Shack (Seattle, Wash.) • Twisted Root Burger Co (Dallas, Tex.)Guy Fieri checks out some places doing America's favorites: Burgers done every which way at a joint run by culinary school grads in Dallas; An Arizona pizza joint serving up family recipes; Plus a Seattle chili shack.