Hello, Hops do different things in your boil when added at different times, that being said here is a generic rundown of what hops do.
60 min in boil is Bittering.
20 to 30 min in boil is for flavor.
5 to 10 min are for Aroma.
There really isnt any fixing adding your hops at the wrong time, but if I had done this in one of my brews I would let it ride and see how it comes out.
Adding your bittering hops at 15 min isn't going to bitter your beer very much at all, its going to give it a lot of hop flavor, and maybe some hop aroma.
I guess you could brew a 1 gallon batch of the same brew and add 1 1/4 oz of nugget at 60 min then add that mix to your current batch to up the bitterness, but this still will not take away the hop flavor that was imparted in the first batch.
As you can see there isn't really any way to make it like the recipe originally called for, but it still might turn out to be a good batch of beer in the end, so ill say it once again, if I had done this in one of my brews I would let it ride and see how it comes out, or maybe over hop a 1 gal batch and add it like I described above.
Worst case scenario, you learned a lesson, we have all made mistakes brewing.
Hope this helps !
Please let me know how it turns out.