01 - Heat the oven to 375°F (190°C).
02 - In a medium skillet, warm a drizzle of oil over medium heat. Cook the chopped onion until softened, about 2–3 minutes, then add minced garlic and sauté for an additional minute. Remove from heat.
03 - In a mixing bowl, blend cream cheese, sour cream, and mayonnaise until smooth and creamy.
04 - Fold in the sautéed onion and garlic, thawed spinach, shredded mozzarella, grated Parmesan, salt, black pepper, and red pepper flakes if desired.
05 - Transfer the mixture into an 8-inch square baking dish. Bake for 20–25 minutes until the dip is hot, bubbling, and the top is lightly golden.
06 - Present the dip immediately with toasted bread, crackers, or fresh vegetable sticks.