Over medium heat in a saucepan, heat the milk with the stock and salt, stirring throughout. Continue to heat the milk mixture over medium heat, and then reduce to low heat once the milk has begun to froth or simmer.
Next, in a second saucepan, heat the butter until melted. Stir in the flour. Reduce the heat to low and stir until well incorporated. Remove from the heat when it is a pale tan color (a blond roux).
Once the roux has thickened, slowly stir into the milk mixture to blend. Continue to stir over low heat until well incorporated. Finally, remove from the heat and whisk in the cheeses. Once mixed, add the pasta, again stir well and serve.