It’s tempting to move the bishop, giving check, and then try to mate with the queen from b7, but the black king can run to f6 and escape. And checking with the queen along the long diagonal allows Black to flee via g8. The answer is the quiet 1. Kh6! and now, no matter which black pawn promotes, White has a ready mate. (If Black runs to g8, then 2. Be6 ends things neatly.)
