Start by portioning your cod into 3-inch sections and poach in the stock for 12 minutes. Remove fish from the poaching liquid and shred into medium flakes. Place in the refrigerator to cool completely.
Peel and cut in half potatoes. Remember to store in a bowl of cold water until ready to steam. Steam potatoes for 20 minutes or until a fork easily pierced the skin.
While potatoes are steaming, dice onion and garlic. Preheat skillet over medium heat and melt butter. Sweat the onions and garlic until the onions are translucent. Transfer to a plate and spread flat. Store in refrigerator until cooled completely.
Once potatoes have steamed, place onto a baking sheet and into a 300-degree oven for 10-15 minutes until steam stops rising. Using the small disc with a potato ricer, rice potatoes. Pat dry with a paper towel and refrigerate until completely cooled.
Once the potatoes and onion/garlic mixture has cooled, combine with the cod in a bowl.
Whisk eggs, mustard, parsley, green onion, salt, and pepper in a separate bowl and combine with cod mixture.
Form patties into 3 inch by 1-inch rounds and lightly coat in panko bread crumbs. Hold on a baking tray in the refrigerator until the oil is ready.
Heat your skillet over medium-low heat. Add 1/2-inch cooking oil into the pan. You'll know the oil is ready when the surface shimmers. Do not let it boil or smoke.
Add in cod cakes two to three at a time being careful not to crowd the pan. Cook 4 minutes on each side and let rest on paper towels once cooked.
While cod cakes are cooking, place remaining uncooked cakes in the refrigerator to keep cold.
Serve immediately or hold warm until you are ready to eat.