Place the tuna in a medium bowl and flake with a fork. Add egg, ¼ cup yogurt, the onion, bell pepper, parsley, 1 clove minced garlic, ¼ teaspoon salt and the pepper. Mix together with a fork just until blended. Shape tuna mixture into 8 small cakes.
Place the breadcrumbs and walnuts in a shallow dish, and gently coat both side of each patty in breadcrumb mixture.
Heat oil in a large nonstick skillet over medium-high heat. Add tuna cakes, in batches if necessary, and cook for 3 to 5 minutes on each side or until golden brown.
While cakes cook, stir together remaining ¼ cup yogurt, the mayonnaise, lemon juice, lemon zest, garlic, and 1/8 teaspoon salt. Serve sauce with tuna cakes.