Place the butter into a large saucepan then cook over a medium heat until melted. Stir in the plain flour then cook out for a minute, stirring constantly.
Add in half the milk then whisk until smooth. Cook out until thick, whisking/stirring regularly. This will take 2-3 minutes over a medium heat.
Pour in the remaining milk & keep cooking until thickened again.
Now add in half the chicken stock & reduce the heat to medium-low. Continue cooking, stirring/whisking regularly until the sauce has thickened back up.
Pour in the rest of the stock then keep cooking until the flour has been cooked out & the sauce has thickened for a final time. It should be slightly thicker than double cream.
Once the sauce has cooked out, add in the mustard & two thirds of the mixed cheese. Stir until melted then take the pan off the heat.
Season the sauce with salt, pepper & grated nutmeg, to taste. Set aside for now.