Belgian Chicken Fricassee (Vol-Au-Vent)

Cook the chicken breasts or the chicken in the chicken stock, until tender (breasts should take about 30 minutes, a whole chicken 45 minutes or more). Drain, but keep the stock. Let the chicken cool down a bit.
In the meanwhile, make tiny meatballs (max. 1 cm across) from the minced meat. Cook the meatballs in chicken stock or water until they are done -- this usually takes just a few minutes, the meatballs will drift to the top when ready. Drain them as well.
Make a "blonde" sauce: melt most of the butter in a medium-sized pot (don't let it become brown), add flour and mix well. Keep the pot on medium heat. Then, very gradually, add splashes (think 2-3 tablespoons) of chicken stock to this mixture. Each time you added some stock, stir very well so that the mixture doesn't become lumpy. You can start adding increasing amounts of stock when your sauce becomes liquidish. Make a sauce that is somewhat thicker than cream; you will probably use most of your chicken stock in this process. This is fine.
Quickly add the raw egg yolk, the lemon juice and the grated cheese to your sauce.
In a large pot, melt the remaining lump of butter and add the mushrooms. Stir-fry the mushrooms on medium heat until they start to "sweat" (release their juices). Season with salt and pepper.
Add the sauce and the ham dices to the mushrooms. Add the meatballs as well. Put on low heat.
With your hands or a knife, pull apart the chicken and make bite-sized pieces (about 1 inch long, 1/2 inch across, or smaller). Add the chicken pieces to your mixture as well.
Ready! Do check if the dish needs a bit more salt or pepper. You can eat the Vol-Au-Vent as it is, with potatoes or rices; you can also pour it in Vol-Au-Vent shells (puff pastry) or even make a chicken fricassee pie (in a puff pastry pie shell).