Fry the onion and the garlic in the oil until softened, about 5 minutes (but don't let it brown.)
All the rest of the ingredients except the olives, stir and bring to a boil (add the juice from the tomato tin as well.) Once it comes to a boil, reduce heat and let simmer covered for 20 minutes or until the beans are tender. Stir occasionally.
Stir in the olives and season with salt and pepper.