Place the fennel, anise, and peppercorns in a dry skillet. Place over medium heat, moving skillet to keep seeds from burning, until seeds are aromatic, about 3 minutes. Transfer seeds and orange peel to a spice grinder; cool slightly. Pulse until finely chopped but not powdery. Transfer to a bowl; stir in the ginger, cloves, cumin, 1 teaspoon salt, and sugar. Set aside.
Place tuna in a glass baking dish, and sprinkle each side with 1 1/2 tablespoons spice mixture. Pat with hands to make it stick. Cover with plastic, refrigerate, and let marinate for 20 minutes.
Make dressing: Whisk the orange juice and vinegar in a small bowl. Whisk in the pepper and remaining 1/2 teaspoon salt. Slowly whisk in 2 tablespoons oil until combined. Set dressing aside.
Heat remaining teaspoon oil in a nonstick skillet over medium heat until hot but not smoking. Add reserved tuna; cook until golden, 3 to 4 minutes. Flip; cook until bottom is golden, 3 to 4 minutes. Tuna should be well browned and crisp on outside, rare to medium on the inside.
Place lettuce, cucumber, onion, and tomatoes on a platter. Slice the tuna, and arrange on greens. Drizzle with dressing, and garnish with feta. Serve.