Preheat your oven to 375°F (190°C). Lightly grease a 9-inch or 10-inch cast iron skillet with olive oil or butter spray.

Crucial Step: Place the thawed chopped spinach into a clean kitchen towel or cheesecloth and wring vigorously over the sink until no liquid drips out. Squeeze the chopped artichoke hearts in paper towels to remove excess brine. This guarantees an ultra-thick, velvety dip that never turns watery.

In a large mixing bowl, beat softened cream cheese, sour cream, mayonnaise, minced garlic, lemon juice, salt, black pepper, and red pepper flakes until completely smooth and creamy. Fold in 1 cup of the mozzarella, 1/3 cup of the Parmesan, and all of the Gruyère cheese.

Stir the wrung-dry spinach and chopped artichokes into the cheese mixture until evenly distributed. Transfer the mixture into the prepared cast iron skillet and smooth the top with a spatula.

Scatter the remaining 1/2 cup of mozzarella and remaining Parmesan evenly across the top. Bake uncovered at 375°F (190°C) for 20 to 22 minutes until hot and bubbling around the edges. Turn on the broiler for the final 2 minutes until the cheese crust is blistered with gorgeous golden-brown spots.

Remove carefully from the oven (use an oven mitt on the cast iron handle). Let rest for 5 minutes to allow the molten cheese to settle slightly, then serve warm straight from the skillet with crunchy tortilla chips, warm garlic pita wedges, or toasted baguette slices.