Prepare crumb crust. Then combine first 6 ingredients in a medium saucepan, whisking well. Bring to a boil over medium heat, whisk constantly for about 8 minutes.
Remove from heat and stir in lemon peel, juice and butter. Pour into pie crust. Cover top of tart with wax paper. Chill at least 3 hours or overnight.
To serve, whip cream and confectioners' sugar until stiff. Spoon or decoratively pipe whipped cream on top of tart.