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 with cooking spray or a little 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. Whisk in the flour and cook for 1-2 minutes until golden.
Gradually whisk in the milk, salt, pepper, and nutmeg. Continue to cook, stirring constantly, until thickened, about 5-7 minutes.
In a large bowl, combine the chopped shrimp, crab meat, spinach, and half of the mozzarella cheese.
Spoon a portion of the seafood mixture onto each crepe, roll them up, and place them seam-side down in a greased baking dish.
Pour the Bechamel sauce over the crepes and sprinkle with the remaining mozzarella cheese.
Bake in a preheated oven at 375°F for 20-25 minutes until bubbly and golden on top.