Combine 1L of water 2 tsp salt in a medium saucepan and bring to a boil over high heat. Slowly add the polenta, whisking constantly. Reduce the heat to low, bringing the polenta to a simmer, and cook, stirring occasionally, until thick, 30mins. Turn off the heat and stir in the butter, cheese and rosemary. Pour the polenta into the prepared dish and cool completely until firm.