Melt the butter in a large frying pan over a low heat. Add the leek and cook for 10-15 minutes until softened. Stir in the flour and continue to cook for a couple of minute.
Pour in the cream and add the grated cheese and mustard. Season with a little salt and pepper.
Meanwhile preheat the grill to high and line a baking sheet with baking parchment.
Place the bread slices on the baking sheet and toast under the grill until golden.Turn the bread over, spoon over the leek and cheese mixture and return to the grill.
Cook until the leek and cheese topping is golden and serve immediately.