First, make the marinade by mixing all the products for it. Put the fish in a small tray and pour on 4 tbsp of the marinade.
Cover and let it sit 30 min.
Heat a grill pan. Put the chopped onions and fennel in a bowl and mix them with the 2 tbsp olive oil. Cook in the pan.
Transfer them to 4 plates and sprinkle on the remaining marinade. Cook the fish in a pan and put it in the bed of fennel and onions. Serve immediately with crunchy bread.