1. Working over a large bowl, cut out segments from orange, then cut into 1/4-inch pieces; squeeze juice from membranes. To bowl, add fennel, tuna, vinegar, and 1 tablespoon oil; season with salt and pepper. Toss well to combine and set aside.

2. In another large bowl, stir together couscous, eggs, and basil until combined and season with salt and pepper. In a large nonstick skillet, heat 3 tablespoons oil over medium-high. Working in batches, drop couscous mixture in 1/4-cup mounds (8 mounds total). With a small spatula, gently flatten each mound and cook until golden and crisp, 3 to 4 minutes, flipping once. Drain on paper towels.

3. Divide tuna mixture among lettuce leaves and sprinkle with fennel fronds, if desired. Serve with couscous cakes.