"This is a typical regional recipe from the middle of the province of Noord-Holland, the Zaanstreek in the Netherlands. The Zaanstreek is situated right above Amsterdam; tourists mostly know it for the mills and old wooden houses at the Zaanse Schans. Here you can find the mill that was the first to make the Zaanse Mosterd (Zaanse mustard). This mustard is coarse and have whole yellow and brown mustard seeds. This is the only kind of mustard you can use for this recipe! When the soup is warm, serve with freshly baked and buttered bread. Eet smakelijk! Good appetite!"
Cook the bacon in a large, deep skillet over medium heat, stirring occasionally, until crisp and evenly browned, about 10 minutes. Pour off the fat and drain the bacon on a paper towel-lined plate.
Melt the butter in a saucepan over medium-low heat. Add the flour and stir until the mixture becomes paste-like and light golden brown, about 5 minutes. Gradually whisk the chicken stock into the flour mixture and bring to a simmer over medium heat. Cook and stir until the mixture is thick and smooth, 10 to 15 minutes. Reduce heat to low.
Beat the egg yolks and heavy cream together until blended. Slowly whisk 1/4 of the hot soup into the cream mixture and return the cream mixture to the soup pot. Whisk in the mustard; stir in the apple and bacon. Cook and stir until the soup is hot but not simmering.