For the involtini: Bring a large pot of salted water to a boil. Choose the best 12 leaves from your savoy cabbage. Separate them, rinse them well and add them, in batches, to the boiling water for 2 minutes. Remove them from the water, quickly rinse with cold water and gently pat dry.
Lay a leaf of cabbage on a cutting board, trim the thick end of the stem and place 2 generous tablespoons of the meat mixture in its center. Roll up the leaf, tucking in the sides, and tie twice with butcher's twine.
For the sauce: Heat the oil in a large nonstick pan over medium-high heat. Add the onion and saute until softened, then add the garlic and cook for 2 more minutes.
Add the tomatoes and lower the heat. Season the sauce with salt and pepper and cook for 15 minutes. Gently lay the involtini in the sauce, distributing them evenly.
Cook, covered, for 30 minutes, stirring gently from time to time and flipping the involtini at least twice.