Break up the tuna in a medium bowl. Stir in the cucumber, olive oil, mustard, scallions, dill, lemon zest, and season with salt and pepper.
Lay the bread on a work surface and spread about 1/4 of the tuna mixture on 4 of the slices. Top with avocado slices and lettuce. Lay the tomato slices on the 4 remaining pieces of bread; season with salt and pepper and drizzle with a bit of olive oil. Close the sandwiches, pressing each one together slightly. Serve.
Alternatively serve them open faced to use less bread.
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