COLD COMFORT Soup, by definition, is comfort food. But it can also be sexy, especially in the summer, when chefs devise recipes that call for ripe fruit. Or it can be edgy, with the imaginative use of fresh herbs. It can even be bracing when it's served chilled. Houston's Mockingbird Bistro offers three smartly updated cold soups : melon gazpacho (garnished with chives), tomato and asparagus (adorned with tomatoes and basil), and cucumber and yogurt with poached Gulf oysters. And if you've ever had the urge to eat guacamole with a spoon, the next best thing—and perhaps a bit more civilized—is the sopa de aguacate that chef Miguel Ravago serves at Austin's Fonda San Miguel. But of all the coolly comforting summer soups, the most luscious is the most traditional—vichyssoise. Dallas' new French darling, Le Rendezvous, makes one of the best versions we've ever dipped a spoon into.