Defrost your frozen spinach and squeeze it dry of any excess liquid. I usually use a large glass bowl to microwave it until it's defrosted. Then, use paper towels or a kitchen towel to squeeze dry the spinach. It has a lot of liquid, and you want to get it as dry as possible. It should look like one big clump when you are done!
Drain the water from the can of artichokes. Cut all the artichoke quarters in half.
Place mayo, yogurt, salt, pepper, garlic, parmesan cheese, and ⅓ of the artichokes in a food processor or high-powered blender. Blend until well combined.
Stir in the spinach, then the remaining artichokes, and finally, ¾ cup fontina cheese.
Spread the mixture evenly into an oven-safe dish. I used a 1.5qt/1.5L dish. Top with ¼ cup of shredded Fontina cheese.
Bake covered with foil for 20 minutes, then uncovered for an additional 20-25 minutes (or until hot and bubbly.)
Turn the oven to broil and let the dip broil for about 5 minutes until it slightly browns on the top. The broiler works quickly, so watch it! Yours could take anywhere from 1-5 minutes!