PUT A LARGE PINCH OF SALT AND SEVERAL GRINDINGS OF BLACK PEPPER IN THE BOTTOM OF A BOWL.
SQUEEZE IN THE JUICE OF 1 LEMON AND WHISK INTO THE SALT AND PEPPER. SLOWLY WHISK IN 1/2 CUP OF OLIVE OIL.
TRIM THE STEMS OF THE MUSHROOMS FLUSH WITH THE BOTTOMS. THINLY SLICE MUSHROOMS USING A CHEF'S KNIFE OR AN ASIAN CLEAVER.
RINSE ANY GRIT OFF THE ENDS OF THE CELERY STALKS. DRY WITH A KITCHEN TOWEL. CUT INTO THIN SLICES.
CUT THE FENNEL BULB IN HALF LENGTHWISE. WITH A PARING KNIFE REMOVE THE TOUGH TRIANGULAR WEDGE AT THE BOTTOM, THEN SLICE THINLY CROSSWISE.
TOSS THE MUSHROOMS, CELERY AND FENNEL TOGETHER WITH THE DRESSING. ALLOW TO SIT FOR 30 MINUTES, THEN TASTE. THE MUSHROOMS TEND TO ABSORB THE DRESSING, SO YOU MAY WANT TO SQUEEZE IN MORE LEMON JUIICE AND ADD MORE OLIVE OIL.
USING A VEGETABLE PEELER, SHAVE ABOUT A CUP OF PARMESAN INTO THE SALAD.
TOSS THE SALAD. GARNISH WITH FRESHLY-CHOPPED PARSLEY AND/OR CHIVES.
SERVES 4-6