Heat the stock in a small saucepan. Then in the other saucepan, melt the butter and add the flour. Stir with a wooden spoon over low heat for 2-3 minutes to create a blond roux.
Make the sauce - Gradually add the hot stock into the pan with the flour and melted butter and constantly stir to blend the ingredients. Bring to a boil, stirring all the time.If you stop stirring or leave the sauce on the heat unattended, it will stick and burn. Lower the heat and simmer until the sauce reaches the desired thickness, about 10-15 minutes.
Season the sauce with salt and a couple of turns of pepper, and allow it to cool for a couple of minutes.
Finally, beat the egg yolks into the sauce one at a time until well incorporated.
Strain the sauce through the sieve into a bowl or another pan.
Season with salt and pepper, lemon juice, and nutmeg.
Serve.