To make the marinade, mix the minced garlic, lemongrass, scallion, ginger, coconut milk, chili pepper (optional) and fish sauce or salt (to taste) in a gallon size resealable plastic bag. Add Kampachi, and coat with marinade. Store in refrigerator for at least 30 minutes. To make the sauce add ¼ cup fish sauce, lime juice, rice vinegar, sugar, crushed garlic, chile pepper (optional), and carrot to a glass jar. Cover the jar with a lid and shake to mix the ingredients together. The jar can be stored in a refrigerator for up to 1 month or until ready to use.
Heat a skillet and coat with cooking spray. Remove Kampachi fillets from bag and shake off excess marinade. Sauté the Kampachi until golden brown on one side, then turn over and let cook for another few minutes.
To serve, arrange fillets on a plate. Spoon sauce over fillet and garnish with your choice of baby greens, thinly sliced Japanese cucumber or bean sprouts.