Finely dice the onion and mince the garlic cloves. Cut the green bell pepper into small, even cubes for uniform cooking.

In a large skillet or wide pan, heat the oil over medium heat. Add the onions and bell peppers with a pinch of salt and sauté for about 10 minutes, stirring occasionally, until softened and slightly golden. If the onions begin to stick, add a splash of water to help them cook evenly. Stir in the minced garlic and cook for another minute until aromatic.

Add the chopped tomatoes along with their juices, followed by the tomato paste, sugar, salt, and black pepper. Stir to combine, ensuring the ingredients are well blended. Reduce the heat slightly and allow the sauce to simmer gently for about 20 minutes, stirring occasionally, until the vegetables are tender and the sauce thickens. Taste and balance the flavor with balsamic vinegar, adjusting the salt and pepper if needed.

Once the sauce has developed a rich consistency, nestle the white fish fillets into the pan, spooning a bit of the tomato sauce over each piece. Let the fish simmer uncovered over low heat for about 5–7 minutes, depending on the thickness of the fillets. The fish should flake easily with a fork when perfectly cooked — tender and juicy but not overdone.

Remove the pan from the heat, sprinkle freshly chopped parsley over the top, and serve immediately. This flavorful dish pairs beautifully with creamy mashed potatoes, rice, or a side of crusty bread to soak up the vibrant tomato sauce.