Heat the oil in a large pan over a medium heat. Add the onion and garlic and cook, stirring frequently, for 3 minutes until softened.
Stir in the tomato purée, sugar, treacle and Dijon mustard. Add the cannellini beans and 350ml water, then cover. Cook over a low heat for 15 minutes until the sauce thickens.
Serve on toasted bread with a sprinkling of fresh chopped parsley.