Heat the oil in a heavy-based saucepan. Add the onion and cook over a low heat for 5 minutes, or until softened. Add the celery and garlic and continue to cook for 1–2 minutes.
Add the tomatoes, stock and soy sauce and bring to the boil, then reduce the heat to a fast simmer and cook for about 15 minutes, or until the sauce begins to thicken.
Add the sugar, mustard and mixed beans and cook for a further 5 minutes, or until the beans are heated through. Stir in the chopped parsley and serve on toast.