Combine 1 tablespoon olive oil and next 5 ingredients in a small bowl. Place tuna in a shallow dish or zip-top bag, and add half of orange juice mixture. Set aside remaining mixture. Turn tuna to coat evenly, and marinate 15 minutes.
Remove fish from marinade, and sprinkle both sides with sea salt and pepper. Heat remaining 1 tablespoon oil in a nonstick skillet over medium-high heat, and sear about 3 to 5 minutes per side, depending on desired doneness. (Tuna is best medium-rare, or very pink in the center, to medium.) Remove from pan; cover loosely with foil, and let stand 5 minutes.
Add reserved marinade mixture and orange sections to pan, and cook just until warmed; keep warm.
Heat 2 tablespoons water in pan; add spinach, sea salt, and pepper; cover and steam 1 minute or just until wilted. Divide spinach and tuna evenly among 4 plates. Pour sauce and drippings over fish.