Melt a knob of butter gently in a good-sized pan. Peel the celeriac and cut it into roughly 4cm cubes. When peeling, remember that the skin is pretty gnarly so cut a little deeper than you would other vegetables. Add these pieces to the melting butter and stir about.
Peel, core and chop the apples and add them to the fray. Turn up the heat a bit. Cook for 25-30 minutes with the lid on until tender, occasionally stirring or adding a splash of water if things appear to be colouring.
Pour the cream into a small pan and stir in the garlic. Simmer gently for about 10 minutes or until the garlic is softened. Set aside.
When the celeriac and apple are pathetically soft, pour in the cream and garlic and stir for a couple of minutes. Transfer all to a blender and blitz beyond recognition. Season.