I N T R O D U C I N G : Cookbook Shelf Salon Taste's new weekly cookbook review column: This week Jim Fobel gives new life to the lowly casserole A L S O : The Surreal Gourmet Tequila shrimp taco with avocado salsa PLUS: How to make a margarita TasteTalk Of tomatoes, innards and the chef's life: The Burnt-out Cook answers readers' questions