In a bowl, swish egg yolks. Set aside.
Melt butter in a saucepan. add flour. Cook for 3 minutes on medium heat, stirring frequently.
Slowly add the milk, whisking to incorporate. Add salt and nutmeg. Cook for 3 minutes, until thickened. Let the mixture cool for 5 minutes. Add in the egg yolks (with feta). Let the sauce stand at room temperature, while the potatoes and meat are cooking.