Slice the onion thinly, julienne the carrot, and slice the celery stalk. Separate celery leaves and cut them into 2cm pieces.

Pat the salmon dry with a paper towel, cut into 4 pieces, and season with salt and pepper. Coat both sides with flour.

Heat vegetable oil in a pan over medium heat. Fry the salmon until golden brown on both sides and cooked through, about 1 minute per side.

In a pot, combine sugar, salt, vinegar, and water. Heat over medium heat until boiling. Add onion, carrot, and celery stalk slices, stir, and bring to a boil again. Remove from heat.

Place the fried salmon in a heatproof container and pour the hot marinade over it. Mix gently. Let cool, cover with plastic wrap, and refrigerate for 1 hour.