Preheat your oven to 400 degrees Fahrenheit.
In a large bowl, beat the egg and then mix in the breadcrumbs or oats and milk.
Add the ground meat to the bowl and use your hands to mix everything together until well combined.
Add the onion, carrot, celery, garlic, salt, pepper, ketchup or tomato sauce, and Worcestershire sauce (if using) to the bowl and mix well.
Add any additional herbs and spices that you like and mix until well combined.
Lightly oil a loaf pan or a baking dish and transfer the meat mixture to the pan or dish, shaping it into a loaf.
Place the pan or dish in the preheated oven and bake for 45-60 minutes, or until the meatloaf is cooked through and the internal temperature reaches 160 degrees Fahrenheit when measured with a meat thermometer.
Let the meatloaf rest for a few minutes before slicing and serving.