Preheat the oven to 180°C. Heat the oil in a large, deep frying pan, add the onions, celery, capsicum and garlic and cook gently for 5 minutes over a moderate heat until soft. Transfer the vegetables to a large bowl.
Add the beef, breadcrumbs and egg to the vegetables, season with salt and pepper and knead the mixture with your hands until well combined.
Put the tomatoes and tomato sauce in a small bowl and mix well. Add half to the meat loaf mixture and mix again.
Transfer the meat loaf mixture to a lightly greased baking dish or roasting tin and shape to make a loaf, measuring about 18 x 25 cm, mounding it slightly in the centre. Starting from the middle of one of the shorter ends, use the side of your hand to make a groove down the length of the meat loaf. Pour the remaining tomato mixture into the groove.
Bake for 1¼ hours, or until cooked through to the centre. (If you have a meat thermometer, it should read 71°C when the loaf is cooked.) Allow to stand for 10 minutes before slicing and serving.