Coat the bottom of the pan with olive oil. Heat the pan on medium-high heat until the oil is shimmering. Add the onion slices and stir to coat the onions with oil. Reduce the heat and stir occasionally. After 10 minutes, sprinkle with salt and then let it cook for 30 minutes, add a little water if the onions look like they are drying out. Once cooked, leave to one side.
Rub salt and black pepper on to the skin of the hake.
Heat 1 tbsp of cornish rapeseed oil in a frying pan, once hot, put the hake skin down into the frying pan. Hold it down so the hake doesn't curl up. Let it cook for three minutes and then turn to the white side. Squeeze lemon over the top.
Add your caramelised onion and add the (washed) samphire to the frying pan for a minute. Once the fish is cooked through, add another squeeze of lemon.
Serve the hake on top of potato cakes (or anything you wish to serve it with) with the onions and samphire.