Place salt cod into a bowl and cover with cold water. Refrigerate for 8 hours; pour off water and replace it with fresh water every 2 hours.

After soaking cod for 8 hours, rinse in cold water and cut into 3-inch portions.

Place cod and potatoes into a large pot and cover with water. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until potatoes are tender and cod flakes easily with a fork, about 20 minutes. Drain and allow to cool for 10 minutes.

Place potatoes and cod into a mixing bowl. Season with seafood seasoning, pepper, and parsley; roughly mash with a potato masher. Stir in eggs and onion until evenly combined; there should still be pieces of cod and potato in mixture.

Place flour into a shallow bowl. Form cod mixture into golf ball-sized pieces and roll in flour. Press between your palms to flatten slightly.

Heat olive oil in a large skillet over medium-high heat. Working in batches, cook cod cakes in hot oil until golden and crispy on both sides, about 3 minutes per side. Drain on a paper towel-lined plate.