In a mixing bowl, whisk together the flour, eggs, milk, melted butter, and salt until smooth. Let the batter rest for 30 minutes.
Heat a non-stick skillet over medium heat and lightly grease it with butter.
Pour about ¼ cup of the batter into the skillet, swirling to coat the bottom evenly. Cook for 1-2 minutes until the edges lift and the bottom is lightly golden. Flip and cook for another minute.
Repeat with the remaining batter, stacking the crepes on a plate.
For the Bechamel Sauce, melt the butter in a saucepan over medium heat. Stir in the flour and cook for 1 minute.
Gradually whisk in the milk, salt, pepper, and nutmeg. Cook, stirring constantly, until the sauce thickens, about 5-7 minutes.
Preheat the oven to 350°F.
To assemble, place a few pieces of shrimp and crab in the center of each crepe, sprinkle with mozzarella cheese, and fold the crepes over the filling.
Place the filled crepes in a greased baking dish and pour the Bechamel sauce over the top.
Bake for 20 minutes until bubbly and golden.