Begin this the night before you are going to make the salad by placing the beans in a bowl and covering them with cold water to soak. Next day, drain the beans, then put them in a large saucepan, cover with fresh water and bring them up to simmering point. Boil for 10 minutes, then cover and simmer gently for 1½ hours, or until tender.
Make the dressing by crushing the garlic and mustard powder using a pestle and mortar. Add plenty of black pepper, the grated lemon zest, juice and olive oil. Whisk everything together very thoroughly, then, when the beans are cooked, drain them, rinse out the saucepan and return the beans to it. Pour the dressing over while the beans are still warm, and stir well.
To serve the salad, arrange three-quarters of the rocket leaves over the base of a serving dish, spoon the beans on top and add the tuna in chunks. Then add the rest of the rocket leaves and top with the onion slices.