In a small bowl, whisk together the avocado oil, tamari, ponzu, serrano, ginger, garlic, green onion, cilantro, lime zest, and lime juice.
Reserve 4 tablespoons of the marinade and set aside for drizzling when serving.
Place the tuna steaks in a shallow dish or zip-top bag and pour the remaining marinade over them. Marinate for 15–30 minutes max, turning once halfway through.
Preheat a grill or grill pan over high heat. Lightly oil the grates or pan.
Remove the tuna from the marinade and gently pat dry.
Sear the tuna steaks for 1 to 1 ½ minutes per side, just until a golden crust forms. The center should remain raw and bright pink.
Slice the tuna and serve immediately, drizzled with the reserved marinade.
