Shred cheeses. Heat spinach in microwave and drain thoroughly. Chop artichoke hearts, reserving liquid.
Sauté onion in butter in large non-stick skillet until softened and translucent. Add drained spinach, sour cream, chopped artichoke hearts, and cubed Philadelphia cream cheese. Cook over low heat, stirring well, until cheese and sour cream are thoroughly blended.
Add most (about ¾) of shredded cheeses and stir until blended. Add Tony Chachere’s Creole Seasoning to taste. Add enough reserved artichoke liquid so that dip is just slightly soupy. Pour into large oven-safe dish.
Bake about 30 minutes until dip is heated through. Remove from oven and sprinkle remaining cheese on top. Return to oven until cheese is golden brown, about 5-10 mnutes. Serve with Tostitos scoops.