Since the advent of the digital camera, we haven't stepped foot inside a Costco. Now we have a reason to return. Today's Hot Stuff ("Catering by Costco") in the Merc rates the appetizers found in the frozen food section of everyone's favorite warehouse store. Some of their top picks are the Full Circle Torta Stella, Cuisine Adventures' spanakopita, Clear Springs' smoked rainbow trout (smoked trout is phenomenal, folks -- miles above smoked salmon), and......