Match the cooking method: oily, steak-like swaps for grilling/pan-searing; milder, flaky swaps for baking; canned options for mix-ins.
Use the listed ratio for your recipe’s “1 portion” amount, then adjust seasoning (salt, lemon, and black pepper) to taste.
If your substitute is lean (cod/haddock), brush or mix in a small amount of olive oil or butter to mimic mackerel’s richness.
If your substitute is salty/strong (sardines/herring), rinse/soak and use the lower ratio so the dish doesn’t overpower.
Cook by doneness: stop as soon as the center turns opaque and flakes easily, then rest 2 minutes to keep texture tender.