Cut the carrots, the celery stalk, the leek very finely (Julienne). This is actually the biggest work.
Clean and finely chop the garlic and shallot.
Peel the potatoes and cut them in half or in quarters, depending on the size. Then submerge them in the chicken stock and season with the thyme and bay leafs.
Cut the chicken breast into 1-2 cm pieces. (one inch)
Heat the butter in a pan and fry the shallot and garlic for a few minutes. Then add the chicken cubes. Fry them over a high heat until they are seared on all sides and lightly browned.
Take the pan off the heat and place the dish with potatoes on it. Bring it to a boil and let it cook for about 5 minutes.
Add the chicken with the garlic and shallot. Season the whole with, for example, nutmeg, salt and pepper.
Then every few minutes, successively add the celery, carrots and leeks to the stew.
Meanwhile, mix the flour with a little cold water and mix the egg yolk with the cream.
Add the flour to the chicken casserole (waterzooi) and let it boil for a few more minutes. The flour binds the sauce. Stir gently so that it is evenly distributed over the waterzooi.
Lower the heat and wait until the chicken casserole stops boiling. Then add the cream with the egg yolk. Keep stirring and then let the ingredients integrate for a few more minutes. Check the seasoning and season further if necessary.
Serve in deep plates and garnish with a sprig of parsley or chervil.