How do you get this pie top?
Typically, you layer the pecans in the bottom of the pie shell, and then pour the custard/syrup over it.  The pecans then float to the top, creating a two layer pie.  You do want large enough pieces to float to the top--whole is traditional just for looking pretty.

Note also that you want to avoid beating air into the custardy/syrupy filling, which will rise to the top and help create the dryer layer above and around the nuts.

See for example, this recipe from Dear Martini, with an embedded video showing the method.