Preheat oven to 375°F (190°C). Lightly grease an 8x8-inch baking dish or a 9-inch cast iron skillet with olive oil or butter.

Crucial Step: Thaw the frozen chopped spinach completely, transfer into a clean lint-free kitchen towel or cheesecloth, and squeeze with all your strength over the sink until zero liquid drips out. The spinach must be bone-dry to keep your dip rich and thick!

In a large mixing bowl, beat softened cream cheese, sour cream, mayonnaise, minced garlic, onion powder, red pepper flakes, salt, pepper, and lemon juice using a hand mixer on medium speed for 2 minutes until completely smooth and fluffy.

Fold in the squeezed dry spinach, chopped artichoke hearts, 1 cup of mozzarella, 1/2 cup of parmesan, and all of the Gruyère with a rubber spatula until evenly distributed.

Spread the mixture into the prepared baking dish. Top evenly with the remaining 1/2 cup of mozzarella and 1/4 cup of parmesan.

Bake at 375°F (190°C) for 20 to 22 minutes until bubbling hot around the edges. Turn the oven to broil for the final 2 to 3 minutes until the cheese crust is deeply browned and blistered in spots.

Let rest for 5 minutes before serving warm alongside warm tortilla chips, toasted baguette slices, or crisp carrot and celery sticks!