Pour 1.5 litres of water into a thick-bottomed saucepan or copper pot, add a level spoonful of coarse salt and bring to the boil. Sprinkle in the flour, stirring with a whisk to prevent lumps from forming, lower the heat and continue cooking, stirring with a wooden spoon, for about 50 minutes.
In the meantime, remove the rind from the Fontina and cut it into small cubes or grate it with a large-hole grater. When the polenta is cooked remove it from the heat, add the Fontina and half of the butter and mix well to melt it.
Turn the polenta out onto a plate or chopping board. Melt the remaining butter and pour it hot over the polenta. Sprinkle with a grinding of pepper and serve immediately.