In a big pot, bring the broth to a boil. Add the Arborio rice after lowering the heat. Cook for 20 minutes, or until the liquid has been absorbed and the rice is tender.
Sauté the onion and garlic in a separate pan until translucent. Add the ground beef or sausage (if using) and cook until browned. Remove from heat and set aside.
Put the cooked rice in a large mixing bowl. Add the grated Parmesan cheese, diced mozzarella cheese, cooked ground beef or sausage (if using), chopped parsley, salt, and pepper. Mix well.
Allow the rice mixture to cool slightly. Then, shape the mixture into golf ball-sized balls, using your hands.
Dip each rice ball into the beaten eggs, then roll it in bread crumbs until evenly coated. Repeat for all the rice balls.
Heat the vegetable oil in a large skillet over medium-high heat. Fry the rice balls in batches until all sides are golden brown. Remove with a slotted spoon and drain on paper towels.
Serve the arancini warm as an appetizer or main dish. They can be enjoyed plain or served with marinara sauce for dipping.