Heat a large non-stick skillet with 3 tbsp. of oil. (6 tbsp. for 4 servings) over medium.
Drain and pat the cod fillets dry. Season both sides with salt and pepper. Slice the fillets into 1 inch pieces. To prepare the batter, mix together the rice flour, spice blend, unbleached flour and 1/2 cup water. (1 cup for 4 servings) Whisk until smooth and you have a pancake batter like consistency. (You may need to add up to 2 tbsp. of water (1/4 cup for 4 servings) to reach the proper consistency.) Coat each cod fillet in the batter, and place into hot oil. Cook for 4-6 minutes per side until golden and fish is cooked through. Remove to a paper towel to absorb any excess oil and season with salt immediately.
Add the juice of 1 lime (2 limes for 4 servings) into a medium to large saucepan. Add sugar and a pinch of salt and heat until the sugar is dissolved. Add the cabbage and red onion. Toss to coat in liquid and set aside to allow the vegetables to pickle.
Next, prepare the dressing. In a small bowl, whisk together the mayonnaise, and half of the garlic. Wash, zest and halve the remaining lime. Add the zest and half of the juice into the dressing, with a pinch of salt. Whisk everything together then taste and adjust seasoning, adding remaining garlic if desired. Wash and mince the cilantro, and wash and halve the tomatoes. Slice the remaining lime into wedges.
To warm the tortillas, heat a large skillet over high. Run a tortilla under cold water so that both sides are wet. Place into the skillet and allow the water to evaporate/steam the tortilla for about 10-15 seconds per side. Remove the tortilla into a clean dish towel and wrap it up to keep warm. Repeat with the remaining tortillas.
To assemble the tacos, divide slaw between corn tortillas. Top with chopped tomatoes, and crispy cod. Garnish with cilantro and lime wedges if desired and enjoy!