PICK THROUGH THE ARUGULA, DISCARDING ANY LARGE, TOUGH STEMS. WASH IN COLD WATER. DRY THOROUGHLY AND WRAP GENTLY IN A TEA TOWEL.
IN A BOWL LARGE ENOUGH TO HOLD THE GREENS, SPRINKLE SALT AND SEVERAL GRINDINGS OF BLACK PEPPER. ADD 1 TBSP OF BANYULS VINEGAR. WHISK TO DISSOLVE THE SALT, THEN BEAT IN ABOUT 1/3 CUP OF OLIVE OIL.
SLICE THE CHERRY TOMATOES IN HALF LENGTHWISE.
TOSS ARUGULA WITH THE DRESSING.
PRE-HEAT A SAUTÉ PAN OVER HIGH HEAT FOR TWO MINUTES.
DRY THE FISH IN PAPER TOWELS. SEASON WITH SALT AND PEPPER.
POUR 1/3 CUP OF OLIVE OIL IN THE PAN. WHEN IT IS SMOKING GENTLY LAY IN THE FILLETS SKIN SIDE DOWN. SHAKE THE PAN SO THE FISH DOESN'T STICK.
COOK FOR ABOUT 2 MINUTES. TOSS IN 3 TBSPS OF BUTTER. BASTE FISH WITH THE BUTTER AND PAN JUICES.
COVER THE FISH AND COOK ANOTHER MINUTE. UNCOVER AND BASTE AGAIN. TEST THE FISH BY INSERTING A SHARP KNIFE IN THE CENTER AND TOUCHING YOUR BOTTOM LIP. IF IT'S WARM, THE FISH IS DONE.
ARRANGE THE ARUGULA ON TWO PLATES. LAY THE COD FILLETS ON TOP AND SCATTER WITH CHERRY TOMATOES. GARNISH WITH LEMON SLICES.
SERVES 2