IN A LARGE SAUTE PAN, HEAT FROZEN SPINACH AND COOK UNTIL SIZZLING AND HOT. SQUEEZE OUT ALL WATER USING A PAPER TOWEL AND SIEVE.
WIPE OUT PAN LIGHTLY WITH A PAPER TOWEL. IN THE SAME PAN, MELT BUTTER AND ADD IN CHOPPED ONIONS. COOK ON MEDIUM LOW UNTIL SOFTENED AND TRANSLUCENT.
ADD IN GARLIC AND COOK FOR ABOUT A MINUTE. TOSS IN SPINACH AND CHOPPED ARTICHOKES AND STIR ALL TOGETHER.
IN A STAND MIXER WITH A WHISK ATTACHMENT OR A HAND MIXER AND BOWL, BEAT CREAM CHEESE UNTIL SMOOTH. SLOWLY ADD IN WHIPPING CREAM A LITTLE AT A TIME, COMBINING TOGETHER BEFORE ADDING MORE. CONTINUE MIXING UNTIL SMOOTH AND NO LUMPS REMAIN.
ADD SPINACH, ARTICHOKE, AND ONION MIXTURE TO THE CREAM MIXTURE AND STIR GENTLY. TOSS IN MOZZARELLA, MONTEREY JACK, AND SWISS, CAYENNE PEPPER, AND SEASON WITH S&P TO TASTES. STIR GENTLY AGAIN UNTIL WELL MIXED.
TRANSFER TO A GREASED BAKING DISH- AN 8X8" OR 2 SMALLER DISHES AND THEN SMOOTH THE TOP OF THE DIP.
COVER THE TOP WITH FRESHLY GRATED PARMESAN, ADD MORE AS NEEDED TO COVER COMPLETELY.
BAKE AT 350 F FOR ABOUT 20 MINUTES UNTIL TOP IS ALL MELTED AND BUBBLING UP.
SERVE WITH TORTILLA CHIPS, SOUR CREAM, AND SALSA.
