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.

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.

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.

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.

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.

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.