For the garlic sourdough crumb topping:
Put the butter into a small saucepan or frying pan and add the garlic, sourdough breadcrumbs and a large pinch of salt. Stir to combine then cook on a low heat until the breadcrumbs turn golden brown. Take off the heat and transfer to a small bowl to cool.
For the mac and cheese:
Melt the butter in a large saucepan then add the flour and cook, stirring constantly, for 2 minutes. Add the milk gradually, stirring after each small addition, to ensure a smooth sauce. Once the milk has all been added, cook for 5 minutes stirring often. Keep stirring while you tip in the cooked pasta and cook for another 2 minutes, then add in the Castello Creamy Havarti cheese and stir until melted. Add extra milk if the sauce becomes too thick - it should be creamy and glossy. Tip straight into a serving dish and top with the garlic sourdough crumb. Serve immediately.