Preheat oven to 375 degrees F

In a large bowl, mix the mayo, sour cream, mustard and onion.

Add the cheeses reserving about 1/8 of the cheese to cover the top, and mix.

Stir in the diced artichoke hearts.

Mix in the fresh spinach and stir well until the mixture is well combined and everything is evenly coated.

Scrape mixture into a large casserole dish and cover with remaining cheese.

Bake for 45-50 minutes, then if desired cook for an additional 1-3 minutes on broil to brown the top of the cheese further.

Let sit approx 15-20 minutes before serving to set.