Heat 30 ml of oil in a large saucepan over medium heat and add onions and saute until softened, about 3 - 4 minutes.
Add beans and cook for about 3 minutes.
Pour in water and bring to the boil, partially cover the pan and simmer over low heat for 60 minutes.
Remove from the heat and puree with a stick blender until smooth.
Return to the heat and stir in the tomato paste, cooking for 5 minutes more.
Season to taste with salt and pepper and stir in the remaining oil.
When served, garnish with the sage and rosemary.