Put milk, cream, mustard, harissa paste and salt in a small saucepan and heat over medium heat, add the cheese slices in pieces, stir again and again in the saucepan.
Finely chop the chillies, add the vinegar and chillies to the melted mixture and remove from the heat. Pour into a bowl, sprinkle with a little hot paprika, cayenne pepper or piementon de la Vera (hot) and serve with nachos and jalapenos.
The sauce also tastes delicious with jacket potatoes and potato wedges.