After Tour of Duty, meet with Elder Maxson for this next quest, which starts off by having you board a Vertibird. If the Brotherhood of Steel is designed to emulate your typical well-armed space marine, then this quest is designed to emulate your standard issue bug hunt. To be specific, it's a hunt for mutants. Start off by taking down the Behemoth from the Vertibird. That will allow the ship to land.
On the ground, take out all the mutants in the immediate area.
The final step is to clean out the inside of Fort Strong. It's just a handful of mutants spread throughout the fort, so be sure to check every room. After the last mutant has been taken out, look for Paladin Danse so you can provide a report. Complete the quest by speaking to Elder Maxson back at the Prydwen.