For the apple-cinnamon puree: Puree the roasted apples and their juices in a food processor or blender. Combine the apple puree with the lemon juice, sugar and cinnamon in a medium pot over medium-high heat. Bring to a boil. Reduce to a simmer and cook until thickened and reduced slightly, about 30 minutes. Cool to room temperature and store in a sanitized jar for up to a month.
For the salted caramel: Heat the sugar in a saucepan over medium heat, until the sugar turns golden. Stir until all of the sugar is dissolved and just starting to turn a lovely amber color.
Turn heat to low, add the butter and stir to combine. Careful, it'll foam up. Then add the heavy cream, vanilla extract and salt, stirring to combine. You'll cook the sauce for 5 to 10 minutes, until it comes together. It'll take quite a bit of stirring! Let caramel cool to room temperature, pour into a glass jar and store in the fridge for up to 2 weeks. Pour over everything and anything!
For the crepes: Pop the flour, both sugars, salt, milk, eggs and melted butter in a large bowl and blend together thoroughly. If at all lumpy, strain.
Heat a large saute pan over medium heat. Lightly coat or spray with vegetable oil and ladle a scoop of the batter into the center of the pan. Tilt the pan, rotating in a circle, to spread the entire crepe out. It should be about 9 inches wide and very thin. Cook until golden brown, about 1 minute per side. Set aside on a plate. Cook the remaining batter until you have about 20 crepes.
For assembly: Place a crepe on a cake platter and spread 3 to 4 tablespoons of the mascarpone over the crepe. Top with another crepe, and spread with 3 to 4 tablespoons of the salted caramel sauce. Top that with another crepe, and spread with 3 to 4 tablespoons of the apple-cinnamon puree. Repeat this process until you've layered 20 crepes.
Pop the crepe cake in the fridge for at least 3 hours (preferably overnight) to set. When ready to serve, remove from the fridge and let it sit on the counter for about 20 minutes to take the chill off. Sprinkle the top with sea salt and enjoy.