1) Trim the leeks wash well and slice finely, discarding the root.
2) Melt the butter over a low heat and add the leeks. Cook, stirring occasionally for 5 minutes. Leave to cool.
3) Preheat the grill to medium, mix together the cooled leeks, grated cheese, spring onions, parsley, salt and pepper to taste. Split the bagels and place under the grill, put some grated cheese on top and return to the grill until bubbling.