Place the wasabi paste and butter in a bowl and mix together well. Refrigerate until needed.
Place the tuna steaks on a flat surface and season with cracked pepper, pressing the pepper firmly but gently into the fish.
Preheat grill to medium high to high heat.
For the Spicy Soy Basting Sauce, whisk together the brown sugar, pickled ginger, soy sauce, rice vinegar, mirin, sake, sambal oelek, green onions, garlic, sesame seeds and sesame oil and set aside.
Place the tuna steaks on the grill and sear for 2 to 3 minutes per side, brushing liberally with the Spicy Soy Basting Sauce. Do not overcook. The tuna is ready when the center is slightly warm but still raw.
Remove from heat and garnish with wasabi butter and Avocado California Rolls