Slice potatoes thin on mandolin or carefully with a Chef Knife.
Slice the leeks into thin half-moons, spray with cooking oil and line the inside of a 9" x 13" cake pan with parchment and build up layers with potato, leek, cream, parmesan and repeat until the pan is full to the top.
Cover pan in parchment, plastic and foil and bake at 350 for 45 min.
Cool with a weighed down pan on top.