Slice the red bell pepper, cut the onion into wedges, and slice the eggplant into 1cm thick rounds.

Mix all ingredients for the sweet vinegar sauce in a bowl.

Cut the mackerel into bite-sized pieces, pat dry with paper towels, season with salt and pepper, and coat with cornstarch.

Heat salad oil in a pan to 170°C and fry the red bell pepper and onion for 2 minutes, then fry the eggplant for 1 minute.

Fry the mackerel pieces for 2–3 minutes, flipping occasionally, until cooked through. Remove and drain excess oil.

Wipe the pan clean, add the sweet vinegar sauce, and heat over medium heat until thickened. Reduce to low heat and add the fried vegetables and mackerel, tossing to coat.