Make the sugo. Heat the olive oil in a wide skillet or shallow saucepan over medium-low heat. Add the smashed garlic and cook gently for 1–2 minutes, until fragrant and lightly golden. Add the passata, season with salt, add the basil and bring to a gentle simmer. Reduce the heat to low and cook for 15–20 minutes while you prepare the polpette.
Combine the polpette ingredients. Add the drained tuna to a large bowl and gently break it into flakes with a fork. Add the mollica, one egg, Parmigiano, parsley, grated garlic, lemon zest, black pepper, and a small pinch of salt. Mix gently with your hands until combined.
Adjust the binding. Firmly press a little mixture in your palm. It should feel soft and moist but hold its shape. If it crumbles, beat a second egg separately and add it a little at a time, mixing gently until the mixture binds. You may not need the entire second egg.
Shape the polpette. Form about 10–12 small polpette, roughly 1½ inches across, compacting them gently between your palms. If they feel especially soft, refrigerate for 15–20 minutes before cooking.
Brown the polpette. Heat enough olive oil to lightly coat the bottom of a separate skillet over medium heat. Add the polpette, working in batches if necessary. Let the first side develop a golden crust before turning, then continue cooking carefully until golden all over, about 5–6 minutes total.
Finish in the sugo. Transfer the browned polpette into the gently simmering tomato sauce. Spoon a little sauce over the tops and cook for another 5–7 minutes, until hot through and set in the center. Avoid vigorous stirring.
Finish and serve. Scatter with chopped parsley and a little extra lemon zest, if desired. Serve warm with plenty of sauce and crusty bread for scarpetta.