Slice the trimmed leeks into thick coins and wash thoroughly in a colander under cold running water.
Melt the butter in a deep sided pan and add the leeks. Cover with a piece of greaseproof paper and a lid and let them cook slowly on a low heat , stirring occasionally, for about 20 minutes until soft.
Toast the bread lightly and place on a baking sheet or grill pan, Warm the grill.
Sprinkle the flour over the leeks, stir and continue cooking for a couple of minutes. Now stir in the cream, mustard and cheese together with the parsley if using. Season.
Once the cream is hot and the mixture has started to bubble, stir in the drained butter beans and continue cooking for a couple of minutes.
Spoon the mixture on to the toast and pop under the grill. Watch carefully and cook until golden then serve.