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. Season both sides of the tuna steaks evenly with salt.
Sear the tuna steaks for 1 to 1½ minutes per side, just until a golden crust forms. The center should remain raw and bright pink.
Slice the tuna and serve immediately as desired, drizzled with the reserved marinade.