Method:

Prepare the Fish: If you're using fresh mackerel, clean and gut the fish, then slice into fillets. If using frozen fillets, defrost them and pat dry with paper towels to remove excess moisture.

Cook the Onion and Garlic: Heat the olive oil in a skillet over medium heat. Add the chopped onion and sauté for about 3 minutes, until soft and translucent. Add the minced garlic and cook for another minute until fragrant.

Prepare the Sauce: Add the canned tomatoes (or fresh tomatoes) to the pan, crushing them gently with a spoon. Stir in the oregano, red pepper flakes, salt, and pepper. Let the sauce simmer for 10 minutes, stirring occasionally.

Add the Fish: Gently place the mackerel fillets in the tomato sauce. Cover and simmer for an additional 10-15 minutes, depending on the thickness of the fillets, until the fish is cooked through and flakes easily with a fork.

Serve: Garnish with fresh parsley, and serve hot with crusty bread or over pasta for a complete meal.

Variations:

For a richer flavor: Add a splash of white wine when cooking the onions.

For a low-carb option: Serve it over steamed vegetables instead of pasta.

For a Mediterranean twist: Add black olives and capers to the sauce.

Tips & Substitutions:

Fish alternatives: If mackerel is not available, sardines or any firm white fish like cod or bass work well in this recipe.

Tomatoes: If you prefer a smoother sauce, you can blend the tomatoes before adding them to the pan.

Dietary needs: This dish is naturally gluten-free and can be made dairy-free as well. Just avoid cheese toppings.