Skate wings with caper butter sauce

I never cook skate wings before. This recipe is from classic French cooking book. I love tender "fish" meat with reach caper sauce and simple boiled potato. Surprisingly, skate wing are very cheap in New Zealand, so I will cook them more often now.

Ingredients:
Serves 4
4 small wings of skate (or 2 big)
300 ml (1/2 pt) water
15 ml (1 tbsp) vinegar
1 small onion, peeled and sliced
bay leaf
salt and freshly ground pepper

Sauce: 
1 shallot, thinly chopped
30 g (1 oz) butter
15 g (1/2 oz) flour
1 tsp lemon juice,
30 g (3 tbsp) capers, chopped
15 g (2 tbsp) fresh parsley, chopped

Method
Put the skate wings in a baking dish and pour over the water, vinegar, sliced onion and bay Leaf. Season with salt and pepper. Bake, covered at 180°C (350°F) Gas 4 for about 15 to 20 minutes. Remove skin from the wings and keep them warm until sauce is ready.

In a saucepan melt the butter, then add shallots and cook them for 2-3 minutes. Add flour stirring well and make up to a sauce with 300 ml (1/2 pt) of the fish liquor, strained from the skate.

Add the chopped capers, lemon juice and parsley.

Pour the sauce over the fish and serve with boiled potatoes and a crisp green vegetable.