Thinly slice the red onion. Slice half the lemon thinly and cut into quarters. Juice the remaining half of the lemon. Remove dill leaves and set aside a small amount for garnish.

Pat the salmon dry with a paper towel and cut into 4 equal pieces. Season with salt and pepper, then coat both sides lightly with flour.

In a bowl, mix the marinade ingredients: sugar, salt, olive oil, water, and lemon juice. Add the red onion and dill, and mix well.

Heat salad oil in a frying pan over medium heat. Place the salmon skin-side down and fry for about 2 minutes until golden. Flip and fry for another 2 minutes until cooked through.

Add the fried salmon and lemon quarters to the marinade. Let cool, cover with plastic wrap, and refrigerate for 30 minutes to allow flavors to meld.