Pat the mackerel fillets dry with a paper towel and season both sides with sea salt and black pepper.
Heat the avocado oil in a large cast-iron skillet over medium-high heat until shimmering.
Place mackerel fillets skin-side down in the skillet and press gently with a spatula to ensure even contact; sear for 3-4 minutes until the skin is golden and crispy.
Flip the fish and cook for another 1-2 minutes until just opaque, then remove from the pan and set aside.
In the same skillet, add the shredded cabbage, minced garlic, and grated ginger, sautéing for 3 minutes until the cabbage is slightly softened but still crisp.
Push the cabbage to one side of the pan, crack the egg into the empty space, and scramble quickly until set before tossing with the cabbage.
Drizzle with coconut aminos and toss everything together for 1 minute to well incorporate the flavors.
Plate the cabbage mixture, top with the crispy mackerel fillets, and garnish with sliced green onions and sesame seeds.