These are super classic and so simple to make. Use leftover rice or thawed frozen rice. For an easy variation, try shredded Fontina or Gruyere cheese.
Preheat the oven to 350ºF. Add about 2 inches of water to a large pot fitted with a steamer insert, and bring to a high simmer. Arrange the peppers in the steamer, cover the pot and cook, rotating the peppers as needed, until they are very tender and pliable, about 25 minutes. Remove the peppers with a slotted spoon, and drain upside-down on paper towels.
Put the oil, onions and a pinch of salt in a large skillet over medium heat, and cook, stirring occasionally, until the onions are soft and translucent, about 8 minutes. Raise the heat to medium-high. Add the beef, garlic, 2 teaspoons salt and a few grinds of pepper, and cook, stirring and breaking the beef up, until browned and mostly cooked through, about 5 minutes. Add the tomatoes, bring to a simmer, then remove from the heat. Let cool in the skillet for at least 10 minutes. Transfer to a large bowl, add the rice, parsley, 1 cup of the mozzarella, breadcrumbs, eggs and Worcestershire and mix to combine.
Stand the peppers up in a 9-by-13 inch baking dish. If they fall over, cut away a little of the bottoms (without cutting through the pepper) to make a flat surface.
Generously fill and pack the peppers with the meat-rice mixture. Top with the remaining 1/2 cup mozzarella. Add just enough water to the pan to cover the bottom to help steam the peppers. Loosely cover with foil, and bake until the peppers are tender and the filling is heated through, about 30 minutes. Remove the foil, and continue to bake for 10 minutes more.