Season both sides of the tuna with salt and pepper. In a small bowl, whisk together 2 tbsp oil, 2 tbsp soy sauce, 1 tsp minced ginger, 2 cloves minced garlic, 1/2 cup chopped cilantro, juice of 1 lime plus the zest and 1 tsp sugar. Add the fish and marinate for at least 2 hours in the fridge. In a small skillet heat the 1 tbsp of oil. Add 4 c. 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 or until desired doneness. Pour the remaining marinade into a small saucepan and reduce until it becomes thick, 2 minutes. Top the wilted spinach with the grilled tuna, followed by 1/2 avocado (thinly sliced) and a nice drizzle of the reduction.