Heat 1 tsp salad oil in a pan over medium heat. Place the salted mackerel skin-side down and cook for 4–5 minutes until browned. Remove excess oil with a paper towel. Flip and cook for another 3 minutes on lower heat until fully cooked.

Thinly slice the red onion and soak in water. Drain thoroughly. Slice the tomato into 5mm thick pieces. Tear the lettuce into 3–4 smaller pieces.

Cut the baguette lengthwise to open it. Toast the inside for 2–3 minutes until crisp. Spread butter evenly on the toasted surface.

Layer lettuce on the bottom half of the baguette. Arrange tomato slices slightly overlapping and sprinkle with salt and pepper. Place the mackerel skin-side up, then add the red onion slices. Close the sandwich.