Heat the oven to 350 F.
Line a rimmed baking pan with foil or spray a 9-by-5-by-3-inch loaf pan with nonstick cooking spray.
Heat the oil in a large skillet over medium heat. Add the finely chopped onion, bell pepper, and celery. Cook until the vegetables are softened, about 2 to 3 minutes. Set aside to cool.
In a large mixing bowl, whisk the egg with the milk; add the breadcrumbs, beef and pork, chopped tomato, Cajun seasoning, salt, garlic powder, black pepper, and Worcestershire sauce. Add the cooled vegetable mixture and blend well with your hands.
Optionally, to check a meatloaf for seasoning, take a small amount and shape it into a flat patty. Fry the patty until it is cooked through and taste it. Adjust the seasonings, as needed.
Loosely pack the meatloaf mixture into the prepared baking pan or loaf pan.
Bake in the preheated oven for 1 hour and 15 minutes.
If desired, spread ketchup or chili sauce over the top of the meatloaf about 10 minutes before done.