Soak the giant beans overnight. 

Boil them in a pressure cooker for 10 minutes to make them a little tender. 

Place the giant beans, the sausage cut into fine pieces without the casing, the leek also finely cut, the olive oil, the tomato paste, salt, pepper and oregano either in a terracotta (preferably) or any other baking dish.

Mix, add water until the ingredients are coverd and bake in a preheated oven for half an hour.