First, preheat oven to 375 degrees Fahrenheit and spray cast iron pan or baking dish with cooking spray.
Prepare the ingredients needed for your meatballs. Dice up the bread. Chop the onion and grate the carrot (if you are using them).
Next, crack the egg in a large bowl. Add in the ground beef and remaining meatball ingredients. Mix everything together until just combined. I prefer to dig in with my hands to mix, however if you choose, you could certainly use a wooden spoon. Be careful not to overmix or it will result in tough meatballs.
Finally, using a tablespoon size cookie scoop (or ice cream scoop), portion out the meatball mixture. Roll the portions between your hands to form round meatballs.
Place meatballs in your cast iron pan or baking dish, arranging them in a single layer.
Bake in 375 degree oven for 24-26 minutes, turning them over half-way through, until they reach an internal temperature of 165 degrees Fahrenheit.