Preheat the oven to 400 degrees.
Pour the pistachios into the food processor and pulse until crushed. The consistency should be the middle ground between coarse and fine. *You don't want a fine powder!*
Remove 2 tbsp of the crushed pistachios and set them aside for your salad.
Add the rest of the crushed pistachios to a large bowl or pan, big enough for you to dip and flatten the fish filets. Set this bowl to the side.
In a separate large bowl, whisk together 1 large egg and 1 tsp of water until thoroughly scrambled. Place this bowl next to the crushed pistachio bowl.
Finally, put 1/2 cup all-purpose gluten free flour into in another large bowl or pan and place that next to the egg wash bowl. The lineup should be: flour, egg wash, then pistachios, in that order. Then, put a plate or cutting board next to the pistachio bowl. You will put each filet onto the plate or cutting board after you coat it in pistachios.
Pat the cod filets dry with a paper towel on all sides. Set them aside next to the flour bowl.
Choose one hand to be your dry hand, and one hand to be your wet hand. With the dry hand, dip the cod filet in the flour until its fully coated on all sides.
Pick up the fish with your dry hand and transfer it to your wet hand. With the wet hand, dip the fish into the egg wash until it's fully coated in the egg mixture.
Place the fish into the bowl of pistachios. Use the dry hand to sprinkle pistachios around the sides of the fish until it's fully coated. Then, put the fully coated fish onto the cutting board. Repeat with the next fillet.
Season a large oven-safe skillet with olive oil on medium high heat, until the olive oil is shimmering. Lower the heat if it begins to smoke, we do not want to burn the crust!
Place the cod onto the skillet and let the fish sear for 30 seconds-1 minute, until one side is light golden brown. Flip and repeat on all four sides of the fish.
Transfer the pan to the oven and let it bake for 5 minutes. When it's done, check the internal temperature of the fish. If it's below 130, put it back in for 2-3 more minutes and continue to check until the temperature is 130 or above. If the temp is 130 or above, remove the fish from the heat and set aside.