Melt the butter in a large saucepan on your stovetop or on the wok burner of your barbecue and add the celeriac, apples and salt, stirring to coat.
Add 2 tablespoons of water and place lid on saucepan, cooking on very low heat for approx. 30 minutes, or until celeriac is tender, checking that the mixture doesn't dry out.
Puree in a food processor or thoroughly with a stab blender.