Cut onions in half rings, bell pepper in cubes (you can also in strips). If a multi-colored bell pepper is not available, you can take a whole bell pepper of the same color. This will not change the taste.
Put cod fillet, onions, bell pepper in a bowl. Add black pepper, salt, vinegar, vegetable (olive) oil. We mix.
I use unscented vegetable oil. You can drizzle lightly with sesame oil before serving. I serve this fish with boiled potatoes.