Season both sides of the tuna with salt and pepper.
In a small bowl, whisk together 2 Tbs. oil, soy sauce, ginger, garlic, cilantro, lime juice, zest and sugar. Add the fish and marinate for at least 2 hours in the fridge.
In a small skillet heat the last Tbs of oil. Add the spinach and wilt slightly. Toss in a teeny pinch of salt and pepper. Remove from heat and set aside.
Remove the fish from the marinade and grill (or sear) for 2 minutes on each side. Keep it rare in the center. PLEASE!
Pour the remaining marinade into a small saucepan and reduce until it becomes thick, 2 minutes.
Top the wilted spinach with the grilled fish, followed by the sliced avocado and a nice drizzle of the reduction.