The trapizzino con polpette al sugo is the build most people meet first, because meatballs in tomato sauce and a pocket of bread are made for each other. The shell is the standard trapizzino: a triangle of pizza-bianca dough baked until the outside crackles and the crumb stays open and chewy, slit along the long side and stood up so it can hold a wet scoop without slumping. The filling is Roman polpette: beef or a beef-and-pork mix worked with soaked bread, egg, grated pecorino, parsley, and a little garlic, rolled small, and simmered through a long tomato sugo until the balls are tender and the sauce has taken on their fat and savour. Two or three meatballs go into the pocket with a spoon of the sauce, and the bread is what turns a plate of polpette into something you eat standing up with one hand.
The craft is the meatball staying soft while the sauce stays tight. A good polpetta keeps a high ratio of soaked bread to meat so it is light and almost spoonable rather than dense, and it is cooked in the sugo long enough to give the sauce body without drying out. The sugo itself is reduced so it clings to the bread and the meat instead of pooling and dissolving the crumb, which is the failure mode of a thin marinara in this format. The trapizzino shell is baked hard at the cut faces so the sauce sits on the surface, the balls are nudged to the closed corner of the triangle, and a final spoon of sugo and a dusting of pecorino go over the top. A torn basil leaf keeps the tomato from going one-note. The sloppy version uses dense overworked meatballs, a watery sauce, and an overfilled pocket that collapses before it reaches the mouth.
The near relations all turn on a ball of seasoned meat in a sauce, and each deserves its own article rather than being crowded in here. There is polpette al sugo served as its own plate, often with the sauce first dressing pasta and the meatballs eaten after, the fried polpette eaten dry with no sauce at all as a different texture entirely, and the trapizzino alla picchiapò which fills the same pocket with shredded boiled beef and sweet onion instead of bound ground meat. Each is a separate argument about meat and tomato, and each is its own subject.