Preheat the oven to 190°C (Gas Mark 5 / 375°F).
Boil the brown lentils in plenty of water, until soft - around 20 minutes.
Meanwhile, heat the oil in a frying pan, and add the chopped onion and garlic. Cook over a medium heat for a few minutes, until soft and fragrant.
Transfer the cooked onion and garlic to a food processor, and add the slice of bread, fresh basil, tomato puree, black olives, and a good pinch of salt and pepper. Blitz to form a chunky paste.
When the lentils are cooked, drain them and add to the food processor. Blitz again until the mixture comes together. It should be well combined, but it's fine to leave a few lentils whole.
Lightly grease a lined baking tray. Roll the lentil mixture into balls, and place on the tray. I made 24 lentil meatballs in total, each measuring around 2 inches in diameter.
Place in the oven and bake for around an hour, or until crispy.
While the meatballs are baking, add the good quality chopped tomatoes to a saucepan, and season to taste. Bring to a gentle simmer, and cook for around 10 minutes, until they form a thick and rich tomato sauce.
When the lentil meatballs are ready, serve them with the tomato sauce and your choice of side (I went for bulgur wheat). Top with the fresh basil.