Place flaked salmon in a large bowl. Add chopped onion, egg, breadcrumbs and mayonnaise. Mix gently using a spoon or hands. Make sure everything combines evenly without breaking the salmon too much.

Add lemon juice, mustard, salt and black pepper into the bowl. Mix again slowly. Taste lightly if needed and adjust seasoning. This step gives the burger a fresh and balanced flavor that makes it special.

Take a small portion of the mixture in your hands. Shape it into round patties. Try to keep them equal in size for even cooking. Place them on a plate and let them rest for a few minutes.

Heat oil in a frying pan over medium heat. Place patties carefully into the pan. Cook each side for about 4–5 minutes until golden brown. Flip gently so they do not break apart.

Toast the burger buns lightly in a pan or toaster. This makes them slightly crispy. Warm buns help improve the taste and hold the filling better without getting soggy.

Place lettuce on the bottom bun. Add the salmon patty and tomato slices. Spread a little mayonnaise if you like. Cover with the top bun. Serve immediately while warm and fresh.