Heat the olive oil in a large frying pan and add the seasoned hake fillets, skin side down. Cook for a couple of minutes until the skin is just beginning to crisp, then add little knobs of butter to the pan around each hake fillet and cook for another couple of minutes until the skin is crisp.
Turn the hake fillets over and cook for another 3-4 minutes until cooked through. This will depend on the thickness of the fillets. Transfer to warmed plates while you make the sauce.
Add the rest of the butter to the frying pan and allow it to gently melt over a moderate heat. When it has melted, add a squeeze of lemon juice and the herbs, swirling to combine. Season to taste. Spoon this sauce over the hake fillets and serve with steamed broccoli and some sautéed new potatoes.