Top and tail the green beans; wash them and, still dripping wet, put them on the stove in a large saucepan with the olive oil; stir them delicately. Peel the onions and clean the carrots; slice them thinly and add them to the saucepan along with the peeled tomatoes. Season with salt and pepper, stir again, put the lid on and cook over a moderate heat for roughly half an hour, stirring frequently. When ready the beans should be soft but firm, not overcooked, and the sauce should be fairly dense.