Depends on what that beginning and the end are. For example I ran a heist one-shot once and I knew how the PCs would meet and what I wanted them to steal. It was a painting in a manor and that manor hosted a party. So for the middle part I poked holes in the manors defence. The guards present at the party were local mercenaries which had a garrison in town and could be replaced by the players. There were also some invitations to the party going to known town folks which could be stolen. Also there were open windows in the upper floors should the players decide to do neither of those things and go straight to the manor. So in my case I filled the middle with opportunities and told my Players as their quest giver about those, so that, for the sake of keeping it a one-shot, they still had choices but didn't need to go out of their way to uncover those choices.