Snap beans and break into 2" pieces, or leave whole if very small. Fill a medium sized pot 3/4 full with water and bring to a boil. Add green beans and cook for 3 minutes. Drain. Mix still warm beans with onion, garlic, salt and olive oil. Leave mixture at room temperature for an hour, in order to let flavors mingle. Immediately before serving, add lemon juice (otherwise the beans turn yellow). Best served the same day.