Place 450 g of room temperature diced fontina PDO in a pan along with a spoon of wheat flour; stir thoroughly with a wooden spoon and add a glass of cold milk. Stir thoroughly and leave to rest for a little while. Heat the mixture on a medium flame, all the while stirring with a wooden spoon, without bringing the fontina PDO to the boil. When the mixture reaches a temperature of 60° and the fontina PDO has melted, remove the pan from the flame and add four egg yolks; whisk the eggs and fontina until thoroughly mixed together. Place the pan back over the flame and stir until heated to 60/70°, so that the eggs cook together with the fontina PDO. As soon as the fondue becomes velvety, serve in raclette trays or very hot soup bowls.