In a clean heatproof jar combine shoyu, vinegar, garlic, and chilies. (Optional: Add fish sauce)
In a small saucepan, bring the two teaspoons and water to a boil over high heat. Pour this over the ingredients in the jar and let cool to room temperature. Taste and adjust the seasoning as needed. Chill in the fridge for at least a day before serving.
Prepare the fish by descaling, gutting, rinsing, and draining it. Make deep incisions across the width of the fish at 1-inch intervals on both sides. Pat it dry with paper towels both inside and out. This helps ensure a crispy skin when frying.
Season the fish with salt and pepper, both inside the cavity and on the outside. (optional: Coat the fish with cornstarch for a crispier texture)
Heat a large skillet over medium-high heat. Add enough oil to cover the bottom of the pan and allow it to heat up until shimmering. You want the oil to be hot but not smoking.
Carefully place the seasoned fish in the hot pan. Allow the fish to cook undisturbed for several minutes on one side until it develops a golden-brown crust. Use a tong to gently lift the fish and check the color. Once the first side is nicely browned, carefully flip the fish over and cook the other side until golden brown and the fish is cooked through. The exact cooking time will depend on the size and thickness of the fish, but it's typically around 4-6 minutes per side.
Once the fish is cooked through and crispy on the outside, transfer it to a serving platter. Serve alongside your chili pepper shoyu for dipping and garnish with green onions lemon wedges. Serve immediately while hot.