Preheat the oven: Begin by preheating your oven to 350°F (175°C). While it’s heating, grease a baking dish (approximately 9-inch square) with cooking spray or butter to prevent sticking.
Prepare the creamy base: In a large mixing bowl, combine 1 cup cream cheese (softened), 1/2 cup sour cream, and 1/2 cup mayonnaise. Using a hand mixer or spatula, mix these ingredients until smooth and creamy.
Add the flavorful ingredients: To the mixture, stir in 2 cups chopped fresh spinach (or 1 cup frozen spinach, thawed), 1 cup chopped cooked shrimp, 1 cup cooked crab meat, 1/2 cup grated Parmesan cheese, 2 cloves minced garlic, 1 tsp onion powder, 1/2 tsp salt, 1/4 tsp black pepper, and 1/4 tsp red pepper flakes if using. Stir until everything is evenly combined.
Transfer to the baking dish: Spread the creamy seafood and spinach mixture into the prepared baking dish, smoothing the top with a spatula.
Bake until bubbly: Place the dip in the preheated oven and bake for 25-30 minutes. The dip should be bubbly and lightly golden on top.
Garnish and serve: Once baked, let the dip cool for a few minutes. Garnish with fresh parsley before serving with your choice of dippers, such as tortilla chips, crackers, or fresh vegetables.