2) Drain the beans; transfer to a 2-litre pot with the diced onion, smashed garlic, bay leaf, thyme sprigs, 1 1/5L of stock and salt and pepper, to taste. Bring to a boil. Lower the heat; simmer until beans are very tender and are breaking apart, about 2 hours. If the mixture becomes dry, add some of the remaining stock. Drain the beans; discard the thyme sprigs and bay leaf. Puree the beans in a food processor, adding a few drops of oil. Adjust seasonings with salt and pepper, to taste, along with the lemon juice, white vinegar and walnut oil, if desired. Reserve.
3) For the olive tapenade, in a food processor or mortar, combine the olives, capers, lemon zest, garlic, parsley and salt and pepper, to taste. Slowly add the oil and pulse or work with the pestle into a coarse paste, retaining some bits of olive and caper for texture. Taste to adjust the seasonings. Reserve.
4) Assemble the sandwich. Smear white bean puree on four pieces of potato bread. Arrange sliced tuna on top. Drizzle tapenade over tuna. Add sliced tomatoes, sliced egg and dressed french beans. Top with greens and toasted bread. Garnish with basil and rocket.