Per Jim Genia, author of Raw Combat: The Underground World of Mixed Martial Arts the definitive history of New York's underground fight scene here are the details on the fight:
Frankie Edgar vs. Erik Uresk. Underground Combat League, July 10, 2005. Venue was an old boxing gym in the Bronx. Frankie was the heavy underdog, and though he won, broke a bone in his face (either from headbutting Erik or getting kneed in the face).
For his second fight with Frankie, BJ Penn actually brought in Erik Uresk to train with.
Edgar would go on to make his official pro MMA debut at Ring of Combat 9 in Asbury Park, N.J. on October 29, 2005 beating Mark Getto by TKO.
Also read: The Man Behind NY's Underground Fight Scene