To make the caper butter, place all the ingredients except the capers into a food processor and blitz to a smooth paste.
Remove from the food processor into a bowl and add the capers. Mix well.
Spoon the mixture onto a piece of cling film and form into a roll. Twist both ends of the cling film.
Place in the fridge until ready to use.
Preheat the oven to 190C/375F/Gas 5.
Heat a griddle pan until very hot.
Season the fish with salt and black pepper and place onto the griddle pan for one minute until just marked.
Remove the fish and turn 90 degrees before placing it back on the griddle pan (this will create a criss-cross griddled appearance).
Cook for a further minute then place the whole griddle pan into the oven for 8-10 minutes until cooked through.
Remove and place on a serving plate.
Meanwhile, heat a frying pan until hot and add half the butter and the broad beans and sauté for two minutes.
Add the mint and season with black pepper, to taste.
Heat a second frying pan until hot and add the remaining butter and potatoes.
Sauté for 2-3 minutes then add the garlic and sauté for a further minute.
Add the parsley and season with salt and black pepper.
To serve, place the fish onto a serving plate and top with several slices of the caper butter. Place under a hot grill to just melt the butter.
Spoon the potatoes and broad beans alongside the fish.
