Make 1.76 oz of Pecorino shavings and finely grate the rest.
In a very large saucepan, bring the milk to a simmer with 1 liter (4.22 cups) of water and some salt. Pour in the polenta and stir constantly over low heat for 3 minutes. Add the mascarpone and grated Pecorino.
Press the polenta into a springform pan lined with parchment paper. Let it cool completely, then refrigerate for 1 hour.
Sauté the sausages in a skillet with 6 tablespoons of olive oil for 5 to 10 minutes, turning often. Remove and cut into thick slices.
In the same skillet, sauté the peeled and chopped onion and the peeled and crushed garlic for 5 minutes. Add the crushed tomatoes, season with salt and pepper, and reduce for 5 minutes.
Return the sausage to the sauce and incorporate half of the basil leaves, chopped.
Preheat the oven to 250°C (482°F). Brush the polenta cake with oil and bake for 5 to 10 minutes, watching for color.
Unmold and cut into portions. Top with sausage in tomato sauce, Pecorino shavings, and basil leaves.