Cut the cod into 1 1/2 inch pieces and place in a bowl with the fish sauce, sugar, pepper, turmeric, garlic, and oil. Mix well and let sit for 10 minutes.
Heat up a non-stick pan over medium high heat. Add a touch of oil to the pan, and when hot, add the fish, being careful not to crowd. When the bottoms are browned and caramelized, about 2 minutes, flip and let cook for another 2 minutes or so, or until cooked through.
Remove from the heat and toss in a bowl with the chopped herbs, tomatoes, and lime juice. Taste and season with salt and pepper.
Serve on charred or warmed tortillas with extra herbs, pickles, lime wedges, and diced red onions, if desired.