Leeks Braised in White Wine & Tarragon with Mustard Vinaigrette.

Heat some butter in a casserole and colour the leeks on all sides over a gentle heat.
Add some chopped tarragon and a splash of white wine and place in an oven to braise.
Remove from the oven and place the leeks on a serving plate - mix the cooking liquior with some whole grain mustard to make a  vinaigrette and pour over the leeks.
This dish can be served hot from the oven slightly cooled or even cold.