Take paper towels and wipe the surface of tuna steaks of any excess moisture.
In a seal-able plastic bag, add crushed garlic, olive oil, rice wine, soy sauce, vinegar, chili peppers, and sugar. Place tuna steaks in the bag and let it marinate for at least 24 hours.
Grilling Tuna Steaks
Turn on the grill to high heat (desired temperature, 400°). Once the temperature reaches 400°, place aluminum foil on top of grill and pour olive oil. Place tuna steaks on the aluminum foil and coat both sides of tuna with olive oil. Place cover and cook one side for 5 minutes. Turn over after 5 minutes, turn off heat, place the cover back on and allow to cook for another 5 minutes.
Take fish off the grill and let it rest for few minutes before serving. Add chopped shiso leaves on top of tuna.