Peel and cube the Russet potato, then boil in salted water until tender, about 10-12 minutes.
While the potato cooks, steam the cod fillet until it flakes easily with a fork, approximately 6-8 minutes.
Drain the potato and mash it in a large bowl until smooth, then allow it to cool slightly.
Flake the cooked cod into the bowl with the mashed potato, ensuring any skin or bones are removed.
Add the egg, sliced green onions, Dijon mustard, half the sea salt, and half the black pepper to the bowl.
Mix the ingredients thoroughly and form the mixture into two thick, even patties.
Heat half of the olive oil in a non-stick skillet over medium-high heat and sear the cakes for 4-5 minutes per side until golden brown.
In a separate small bowl, whisk the remaining olive oil, lemon juice, apple cider vinegar, and remaining salt and pepper.
Toss the mixed baby greens in the dressing and serve immediately alongside the warm fish cakes.