Make the cucumber layer: Drain the yogurt in a coffee filter-lined sieve set over a bowl for 1 hour. Put the cucumber in another sieve, sprinkle it with the salt, and let it drain for 1 hour. Pat the cucumber dry between paper towels and transfer it to a bowl. Stir in the yogurt, garlic, and salt and pepper, to taste, and combine the mixture well. Set aside
Make the bean layer: Heat the oil in a skillet over medium heat. Add the onion, garlic, oregano, and cook, stirring, until softened. Add the wine and simmer until reduced by 1/2.
In a food processor, puree the beans and the onion mixture with the broth and season with salt and pepper. Transfer the mixture to a bowl and chill.
Make the tuna layer: In a bowl toss together the tuna, onion, olives, parsley, vinegar and olive oil. Season with salt and pepper.
Assemble the salad: Spread the bean puree in the bottom of a trifle bowl, top with the tuna mixture, and spoon the cucumber mixture over the top. Chill covered, for at least 3 hours or overnight.
When ready to serve, sprinkle the top with the garnish and serve.
Recipe courtesy of Gourmet magazine