Put milk, half of the sugar, and cinnamon in a pan and heat it until boil.
In a bowl, mix the poppy seeds with cornstarch, then add to the hot milk and quickly mix.
Cook this mixture for about 3-4 minutes to thicken and cook the cornstarch, whisking constantly, on medium heat.
Whisk the egg yolk together with oil.
Remove the poppy seed filling from the heat and leave it to cool for a minute or two. Then, add the egg yolk mixture, mix well and set aside to cool completely. You can start making the lemon cheesecake filling and then come back to finish this.
Make the meringue: beat the egg white until frothy and then slowly start adding the other half of the sugar while mixing. Mix for about 1-2 minutes until you achieve stiff peaks. When the poppy seed mixture cools down to room temperature, gently fold in the meringue.
Take the baking pan with the dough from the fridge and distribute the poppy seed filling on top.