01 - Preheat your oven to 350°F.
02 - Slice the top off the sourdough loaf and hollow out the center, leaving a 1-inch thick shell. Reserve the removed bread for dipping.
03 - In a medium mixing bowl, combine cream cheese, sour cream, mayonnaise, mozzarella, and Parmesan. Mix until smooth.
04 - Add spinach, artichoke hearts, garlic, salt, pepper, and red pepper flakes (if using). Stir until well combined.
05 - Spoon the mixture into the hollowed bread bowl. Place the filled bread bowl on a baking sheet.
06 - Bake for 25-30 minutes, until the dip is hot and bubbly, and the top is lightly golden.
07 - During the last 10 minutes, arrange bread pieces around the bowl to toast.
08 - Serve hot, using bread pieces to dip.