In a large bowl, combine water, soy sauce, salt, green onions, garlic, ginger, black pepper, and red pepper flakes. Stir until the salt fully dissolves. For best results, dissolve the salt in 1 cup of hot water, then allow the mixture to cool before adding the remaining ingredients.

Submerge the tuna steaks completely in the brine. If using a resealable plastic bag, ensure all the tuna is evenly coated. Refrigerate for 1-3 hours—brining too long may result in overly salty tuna with a mushy texture.

Remove the tuna steaks from the brine, discarding any excess. Gently pat the fish dry using paper towels. For optimal smoking results, allow the tuna to rest in the refrigerator for at least one hour to form a pellicle, which enhances smoke absorption.

Preheat the smoker to 200-225°F (93-107°C), following the manufacturer’s instructions. Place the tuna steaks directly onto the smoker grates, optionally brushing them with olive oil to maintain moisture.

Smoke the tuna for 1-1½ hours, or until the internal temperature reaches 145°F (63°C) and the flesh flakes easily. Smoking time may vary depending on the thickness of the steaks and the smoker’s temperature stability.

Once the tuna reaches the desired doneness, remove it from the smoker and allow it to rest for 5 minutes before serving. Serve warm as a main dish, or use it as a flavorful addition to salads, sandwiches, or dips.