Straight on till Morning (слэш)

Tony Stark resigned his commission in Starfleet five years ago, after a disastrous away mission, and he swore he'd never go back. He just wants to be left alone to build warp engines in peace. But the universe has more in store for him than that, as he discovers when Admiral Fury comes to him with an offer he could never have expected and cannot possibly refuse: first officer and chief engineer aboard the all-new USS Avenger, a starship of Tony's own design. What's more, the Avenger's captain is Steve Rogers, hero of the Earth-Romulan War. Believed dead for over a century, Steve is miraculously alive... and very, very attractive.
But nothing is ever easy for Tony. As he wrestles with his secret desire for his new captain and his not-so-dormant fears, another mission starts to go wrong, and Tony becomes aware that Steve has secrets of his own -- and the truth could change everything.