Place the potatoes in a large saucepan of cold, salted water. Bring to a boil over high heat and then reduce to a rapid simmer, cooking until tender to the tip of a knife, 15-20 minutes.
Meanwhile, place the cod in a high-sided skillet and cover with water. Bring to a simmer over moderate heat, cooking until opaque and starting to flake, about 5 minutes.
Drain well; flake the flesh into a large mixing bowl and set aside. Wipe the skillet clean with paper towels.
Set the skillet back over medium heat and add 2 Tablespoons oil. Add the garlic, fry for 30 seconds, then stir in the crushed tomatoes, red pepper flakes, ¼ teaspoon salt, ⅛ teaspoon black pepper, and a splash of water.
Bring to a simmer and cook until slightly thickened, stirring occasionally, 8-10 minutes. Set off the heat when ready.
Drain the potatoes when ready. Return them to the saucepan they were cooked in and let steam off for a few minutes; mash until smooth.
Add the mashed potato, breadcrumbs, half the parsley, Parmesan, eggs, ½ teaspoon salt, and ¼ teaspoon of black pepper to the bowl with the cod. Thoroughly combine with your hands.
Divide and shape into five or six cakes; dust with the flour, shaking off any excess.
Heat a large non-stick skillet over moderate heat until hot. Add the remaining 2 Tablespoons oil and then position the cod cakes in the skillet, spaced apart. Fry until golden brown all over, flipping once, about 3 minutes per side.
Meanwhile, reheat the tomato sauce over low heat. Transfer completed cod cakes to the sauce, serving straight away with the remaining parsley as a garnish.
