Make the flaky spelt pie pastry and follow the single blind baked pie crust instructions via my recipe linked in the Notes below.
In a medium saucepan whisk together the sugar, cornstarch and salt. Add the milk in gradually while whisking gently. Cook on medium heat, whisking constantly until mixture is bubbly and thickened. Cook for 2 more minutes and then remove from the heat.
Whisk a small quantity of the hot mixture into the beaten egg yolks and then immediately add the egg yolk mixture to the rest of the hot mixture. Whisk and cook for 2 more minutes. Remove from the heat, add the butter and vanilla extract and then whisk until smooth. Set aside.
Add the lemon juice to a medium bowl then slice the bananas, add them to the bowl and gently stir so all the slices are coated in lemon juice. Add these to the pudding mixture, fold together gently and then pour it into the cooled and baked pie pastry crust. Line the top of the pudding with plastic wrap so it doesn't develop a skin then chill thoroughly, overnight is best.
Before serving, whip the whipping cream and sugar to stiff peaks and spread or pipe on top of the chilled pie.