In a large frying pan, saute the onion and garlic in the olive oil until translucent.
Add the green beans and enough water to cover. Season and leave to simmer until the water has mostly evaporated and the beans are tender.
Add the tomatoes, salt and pepper (a pinch of sugar too, if the tomatoes are too tart) and simmer 20 minutes or so until the sauce is thick and rich and the beans are soft. They are meant to go that lovely olive-green colour so do not be worried thinking you've overcooked them! They are at their best, trust us.