Blanch the garlic cloves for 1 minute in boiling water. Drain and peel. Boil or steam the potatoes and while they are cooking melt 2 oz (50 g) of the butter and add the garlic cloves. Crush them with a wooden spoon in the butter. Let them soften over a gentle heat for about 7 minutes and then stir in the flour, nutmeg and mustard. Pour over the hot milk, blending to make a smooth sauce.
Purée the cooked potatoes, stir in the other 2 oz (50 g) of the butter and spoon by spoon, mix in the garlic sauce. Finally, stir in the cream.