Heat 2 tablespoons of oil in a large pan over medium heat. Add sliced Portobello mushrooms, sprinkle with salt, and cook for 5 minutes, stirring and tossing mushrooms until they get soft and release juices. Add cooked chicken, stir with mushrooms for 1-2 minutes to warm it up. Season lightly with salt and pepper. Remove from heat and keep covered.
In a medium pan or a skillet, melt 2 tablespoons of butter, add 2 tablespoons of flour, cook and stir for about 1 minute to form a roux.
Pour milk into the hot flour-butter mixture gradually, while whisking. Continue cooking the mixture, constantly stirring, on medium or high heat until the mixture bubbles and thickens – about 1 or 2 minutes. Add ¼ teaspoon nutmeg, Parmesan cheese, and gradually add ¼ or ½ of chicken bouillon cube, to taste. Don't add a whole cube right away, just add enough to make the sauce flavorful.
Add the white creamy sauce to the mushroom-chicken mixture. You don't need to add all of the white sauce to the mushrooms - add just enough to coat the mushrooms and chicken. You can always add more white sauce if needed. Stir to combine. Season with salt and pepper if necessary.
There are 2 ways to fill the crepes. Place some of the mushroom mixture in the center of the crepe vertically in a shape of a thick line, then fold two opposite sides of the crepe inwards - like you see on the photos. Or, place some of the mushroom mixture on half of the crepe, then fold the other remaining half of the crepe over on top.
Sprinkle the top of crepes with chopped green onions or chives.