- In a heavy soup pot over high heat, combine the tomatoes, onion and garlic cloves. Bring to a boil, reduce the heat to low and simmer, uncovered, until the sauce thickens, 45 to 60 minutes.
- Remove from the heat and discard the onion. Pass the mixture through the fine disk of a food mill placed over a clean saucepan. Alternatively, puree in a food processor fitted with the metal blade, then pass through a fine-mesh sieve. Season with salt and pepper.
- Meanwhile, prick the sausages in several places with a fork. In a large saucepan, bring 1/2 cup of the water to a boil. Add the sausages and cook over medium-high heat, turning occasionally, until golden on all sides and half-cooked, 10 to 12 minutes. Drain and, when cool enough to handle, cut each sausage on the diagonal into 2-inch pieces.
- In a large, heavy saucepan, bring the remaining 8 cups water to a boil over high heat. Slowly add the polenta in a steady stream while whisking constantly. Continue to whisk until the mixture thickens, 3 to 4 minutes. Switch to a wooden spoon, reduce the heat to medium and continue to simmer, stirring, until the polenta pulls away from the sides of the pan, 20 to 25 minutes.
- Stir in the rosemary and butter. Season with salt and pepper.
- Meanwhile, place the pureed sauce over medium heat and bring to a simmer. Add the sausage pieces and the olives and simmer, uncovered, until the sausages are cooked through, about 10 minutes.
- Divide the polenta equally among warmed individual plates. Spoon the sauce and sausages around the edges.
- Using a cheese shaver or a vegetable peeler, shave a few pieces of the cheese over each serving. Garnish with the parsley and serve immediately.