Identify your cook method: pan-sear (needs firm flakes), bake/roast (needs steady tenderness), or gentle steam (needs delicate flake).
Pick the closest texture in the guide, then swap at a 1:1 fillet amount.
If your substitute is softer (tilapia/sole), reduce total cook time by about 10–25% and check for flaking sooner.
If your substitute is drier (cod), increase moisture support (foil, a splash of stock, or a pan sauce) and pull earlier.
Season to the fish: add salt to balance mildness; if it tastes stronger, brighten with lemon and a small pinch of sugar.