Prepare the Crepe Batter:

• In a blender or using an immersion blender, combine the flour, milk, eggs, butter, nutmeg, salt, and pepper. Blend until smooth.  
  
• Let the batter rest for at least 30 minutes. For best results, refrigerate for a few hours or overnight.

Cook the Crepes:

• Heat a non-stick skillet over medium heat.  Grease the skillet with a small amount of melted butter. 
  
• Pour about 3 oz/ 90ml of crepe batter into the skillet, swirling it to coat the bottom evenly.  
  
• Cook until the edges start to lift and the bottom is lightly golden. Flip and cook briefly on the other side.  
• Repeat until all the batter is used. Stack the crepes on the top of each other on a plate, immediately cover with a plastic wrap to keep warm and to avoid the crepe from drying.

Prepare the Filling:

• In a separate non stick fry pan, sauté the onions until caramelized.
• Add the mushrooms and cook until they release their moisture.
  
• Stir in the spinach and cook until wilted and season with salt and pepper.  
  
• In another pan (or the same pan) add the chicken pieces and cook until they're no longer pink and have a nice color.
  

Make the Cheese Sauce:

• In a saucepan, melt the butter over medium heat.  
• Stir in the flour to create a roux and cook for a couple of minutes until lightly golden.  
  
• Gradually whisk in the milk until smooth. Cook, stirring constantly, until the sauce thickens.  
  
• Reduce the heat to low and add the grated Gruyere and Parmesan cheeses. Stir until melted.  
  
• In a small bowl, beat the egg yolks and quickly add them to the sauce to avoid scrambled eggs. 
• Cook for a minute or two to thicken further. Season with salt and pepper.

Assemble the Chicken Crepes:

• Preheat the broiler.  
• Lay a crepe flat inside an individual serving dish.  Spoon a portion of the chicken-mushroom-spinach mixture onto the center of the crepe.  
  
• Ladle some of the cheese sauce over the filling.
  
• Fold the sides of the crepe over the filling and then fold in the top and bottom to form a rectangular packet.  
• Add more sauce on top of the crepes.  Top with Mozzarella cheese.
  

Broil and Serve:

• Place the dish of crepes under the broiler for a couple of minutes until the cheese is melted and bubbly, and the crepes are golden brown. 
  
• Remove from the broiler and let cool slightly before serving.  Serve the chicken crepes warm, garnished with chopped parsley if desired.
  
• If you make them in advance and refrigerate them be sure to cover them with plastic film to avoid the sauce from drying.  
• To serve them bake them in a 350ºF/175 ºC until internal temperature reaches at least 150ºF/190ºC.  This could take 20 to 30 minutes.

Enjoy your delectable Cheesy Chicken Crepes, a throwback to the 70s with a modern twist!