Cook the com on the cob in boiling water for 8-10 minutes, until tender.
Using a sharp knife, remove the kernels and set aside.
Stir the cornmeal into 250ml (8fl oz) of the water to make a batter.
In a large heavy pan, bring the remaining water and salt to the boil.
Add the commeal batter all at once and stir continuously until the mixture is well blended.
Add the chopped rosemary.
Reduce the heat so that the mixture is simmering and stir constantly for 10-15 minutes, until the polenta pulls away from the side of the pan.
Add the cooked corn kernels.
Pour the polenta into a 25cm (10 in) pie plate, smooth the top with a spatuta and leave it to cool.
Once set, cut the polenta into slices and fry in a little olive oil, until slightly crisp.
Alternatively, brush with oil and bake or grill.
Serve with tomato sauce or ratatouille.