Finely chop 2 cloves garlic and place in a bowl. Add 2 tablespoons sweet soy sauce (Ketjap Manis), 1 tablespoon balsamic vinegar, and 2 tablespoons oil. Stir until well combined and season with salt, pepper, and dried crumbled chilli pepper to taste.
Place the 4 tuna steaks in the marinade and coat well on both sides. Cover and refrigerate for 1 hour.
Heat a grill or frying pan over medium-high heat. Grill or fry the tuna steaks for 2-3 minutes per side until seared.