A Rose of Steel by Katherine MacDonald

Surviving an assassination attempt, Dr Asami Thorne is saved by a mysterious “beast”—a former soldier left disfigured by the very government trying to kill her. Escaping to the underground beneath the great clockwork city of Petragrad, Asami refuses to spend the rest of her life in the abandoned railroads. She works alongside Beast to uncover the truth behind the experiments that destroyed his life and threaten hers.
But untold horrors await the both of them, and Asami risks far more than losing her life when she finds herself falling for her mysterious saviour…