Prepare the Mackerel:
Rinse the mackerel fillets under cold water and pat dry with a paper towel. Season both sides with salt and pepper.
Heat the Pan:
In a large frying pan, heat the olive oil over medium heat. Once hot, add the chopped onion and sauté until translucent, about 5 minutes.
Add Garlic:
Stir in the minced garlic and cook for another minute until fragrant.
Sear the Mackerel:
Gently place the mackerel fillets skin-side down in the pan. Cook for 4–5 minutes until the skin is crispy. Carefully flip the fillets and cook for an additional 4–5 minutes on the other side. Remove the fish and set aside on a warm plate.
Create the Sauce:
In the same pan, pour in the red wine, scraping up any brown bits from the bottom. Add the red wine vinegar, capers, and oregano. Bring to a boil and let it simmer for about 10 minutes, allowing it to reduce and thicken.
Combine and Serve:
Return the mackerel to the pan, spooning the sauce over it. Let it cook for an additional 2–3 minutes to heat through. Serve garnished with fresh parsley.