Preheat the oven to Gas Mark 7, 220°C, fan 200°C. Combine the chives, tarragon, parsley, breadcrumbs, lemon zest, 2 tbsp olive oil and a little salt and pepper in a bowl.
Drizzle the fish on both sides with half the remaining olive oil and season well. Arrange the fish in a shallow baking dish and divide the breadcrumb mixture evenly between the fillets. Gently press down with your fingers to form a tightly packed crust. Drizzle with a little more olive oil and cook on the top shelf of the oven for 10-12 minutes, until the crust is golden and the fish is cooked through.
For the tarragon mayo, mix all the ingredients in a small bowl until combined. Serve the mayo along with the fish, some lemon wedges and crispy fried potatoes on the side, if you like.