For the herby butter: Set aside a tablespoon of butter. Place the remainder along with the garlic, tarragon, parsley and chives in a food processor, season and blend until thoroughly mixed.
To prepare the fish: Brush the reserved butter over a grill tray. Place the fillets on the tray, skin side down, season with a little salt and black pepper, then spread over the herby butter. Sprinkle over the bread crumbs. Place in the fridge for ten minutes to allow the butter to harden up.
To cook the fish: Heat the grill to high. Place the grill tray under the grill and cook for about 4-5 minutes until the fish is cooked.
To cook the spinach: While the fish is cooking, heat some olive oil in a wide frying pan and add the spinach. Allow to cook, turning frequently, for about a minute or until wilted. Season.
To serve: Divide the spinach between four serving plates. Place the whiting fillets on top. Spoon over any butter from the grill tray. Serve with a wedge of lemon to squeeze over and some baby potatoes.