Fish pie with crispy gnocchi topping

Method

1. Put the fish, petits pois and milk in a shallow ovenproof pan or casserole and cover with a tight-fitting lid. Bring to a simmer over a medium heat, then turn the fish over, re-cover and simmer for 2 minutes until just cooked through. Set a sieve over a jug, strain the fish and peas, then set both aside.
2. Heat the grill to medium-high. Return the pan to the hob and melt the butter, then whisk in the flour and cook over the heat for 2 minutes. Gradually whisk
   in the reserved cooking liquid until smooth, then bubble for 2-3 minutes to thicken. Whisk in the mustard, 1½ tsp tarragon, lemon juice and freshly ground black pepper. Return the fish and peas to the pan and stir gently to coat in the sauce.
3. Toss the gnocchi with the olive oil. Scatter over the fish and grill for 5-8 minutes until golden and crisp. Serve topped with the lemon zest, remaining tarragon leaves and steamed green beans.