01 - Preheat the oven to 375°F (190°C).
02 - Heat a drizzle of oil in a medium skillet over medium heat. Sauté the chopped onion for 2 to 3 minutes until softened, then add minced garlic and cook for an additional minute. Remove from heat.
03 - In a large mixing bowl, blend the softened cream cheese, sour cream, and mayonnaise until smooth and uniform.
04 - Fold in the sautéed onion and garlic, thawed spinach, shredded mozzarella, grated Parmesan, salt, black pepper, and optional crushed red pepper flakes until evenly distributed.
05 - Spoon the mixture into an 8-inch (20 cm) square baking dish or a similar-sized ovenproof container.
06 - Bake in the preheated oven for 20 to 25 minutes, until the dip is hot throughout and the top is lightly golden.
07 - Serve immediately alongside toasted bread, crackers, or fresh vegetable sticks.