TRIM,CLEAN,AND DRY MUSHROOMS AND SLICE THEM IN 0.125 INCHTH IN.PLACE IN AN ENAMLED OR NON STICK SAUCEPAN WITH THE OIL,ONION,GARLIC.COOK UNCOVERED,STIRRING OCCASIONALY,FOR FIVE MINUTES.ADD THE TOMATOES,BASIL,PARSLEY,SALT AND PEPPER AND COOK,UNCOVERED,10 MINUTES LONGER.STIR IN VINEGAR AND CAPERS.GARNISH WITH SLICED TOMATOES,AND MUSHROOMS.SERVE HOT OR AT ROOM TEMPERATURE.SERVES 6 TO 8.