In a saucepan of medium-high heat, sauté fresh sausage with 2 tablespoons olive oil and clove of garlic. Cook until brown. Discard garlic clove.
Pulse sausage, mortadella and pancetta, separately. Combine and set aside.
Steam spinach, just until wilted. Drain well and chop roughly.
Have loaf pan greased and ingredients prepared, as polenta will start to harden quickly.
Bring 4 1/2 cups of water to a rapid boil. Salt to taste. Add polenta to water by whisking in slowly. Stir in 1/2 cup of grated Parmesan and 1/4 stick of butter. Cook until mixture has thickened about 1 minute.
Layer in loaf pan: 1/3 polenta mixture, sprinkle with 1 cup mozzarella, meat mixture, then top with spinach. Top with another 1/3 of polenta mixture, then remaining mozzarella. Cover with the rest of the polenta and chill for 1-2 hours or until set.
To serve: preheat oven to 375Ëš. Flip loaf pan to remove terrine, then slice. On a shallow, non-stick baking sheet, arrange slices and drizzle with olive oil. Bake for 8-10 minutes, or until warmed and cheese melted.
Serve with a salad of torn burrata and cherry tomatoes in olive oil.