Preheat the oven to 375°F. Grease a 9” cast-iron skillet with a light coating of oil.
Combine the mayonnaise, Dijon mustard, minced garlic, salt, cayenne pepper, and parmesan cheese in a large mixing bowl. Whisk until smooth.
Pat the artichoke hearts completely dry and roughly chop each quarter into 3-4 pieces. Fold the artichokes into the mayonnaise mixture.
Add the mozzarella cheese, fontina cheese, and spinach to the bowl. Fold all the ingredients together (I find it easiest to wear a pair of gloves and use your hands).
Spread the dip into the skillet and flatten it into one, even layer. Bake for 25 minutes, until bubbling and slightly golden. Serve with the scallion relish on top and crackers on the side for dipping.
Combine the scallions, parsley, lemon juice, and lemon zest in a bowl.
Mix until incorporated and serve on top or alongside the dip.
This dip is also amazing served with toasted bread!