This dish couldn’t be easier to make – it takes about 20 minutes to prepare and 20 minutes to cook. You can prepare this gratin the day before and then just pop it in the oven 20 minutes before you want to serve it. Fennel is the perfect partner for white fish as the aniseed taste lifts the flavour and the dill adds so much freshness. I love serving this dish with fluffy basmati rice and a green leaf salad.