Prep the Slow Cooker & Spinach: Lightly grease the interior of a 3 to 4-quart slow cooker with nonstick olive oil spray. Thaw the frozen spinach and squeeze it tightly inside a clean linen dish towel to extract every drop of moisture.

Combine Ingredients in Slow Cooker: Place the cubed cream cheese, sour cream, mayonnaise, minced garlic, lemon juice, kosher salt, black pepper, and red pepper flakes directly into the slow cooker. Add the squeezed dry spinach, chopped artichoke hearts, 1 cup of the mozzarella, and 3/4 cup of the parmesan.

Slow Cook on LOW: Cover with the lid. Cook on LOW for 1.5 to 2 hours (or on HIGH for 1 hour). At the 1-hour mark, remove the lid and give everything a vigorous stir with a wooden spoon until the cream cheese melts smoothly into the greens and cheeses.

Top with Cheese & Melt: Smooth the top with a spatula. Sprinkle the remaining 1/2 cup mozzarella and 1/4 cup parmesan evenly over the surface. Cover and cook for an additional 20 minutes until the top cheese layer is fully melted, bubbling, and gooey.

Switch to WARM & Serve: Switch the slow cooker to the WARM setting to keep the dip velvety for party guests throughout the evening. Garnish with chopped parsley and serve with warm tortilla chips and crunchy vegetables!