Cook the granulated sugar and water in a saucepan over medium heat without stirring until deep amber. Remove from the heat, carefully whisk in the warm cream, then whisk in the butter and salt. Cool the caramel until thickened.
Cook the apples and butter over medium heat for 4 minutes. Add the brown sugar, cornstarch, cinnamon and lemon juice, then cook for 3 to 5 minutes until the apples are tender and glazed. Cool completely.
Heat the oven to 400 F and line a baking sheet with parchment paper. Bring the water, milk, butter, sugar and salt to a boil, then add the flour all at once and stir vigorously until a smooth film forms on the bottom of the pan.
Transfer the dough to a mixing bowl and cool for 5 minutes. Beat in the 4 eggs one at a time until the dough is glossy and slowly falls from the beater in a thick ribbon.
Pipe the dough into a thick 8-inch ring, adding a second ring directly beside it and a third ring centered on top. Brush with the beaten egg and sprinkle with sliced almonds.
Bake for 15 minutes, reduce the oven to 350 F and bake for another 30 to 35 minutes until deeply golden and firm. Turn off the oven, open the door slightly and leave the ring inside for 15 minutes before cooling completely.
Beat the mascarpone, heavy cream, powdered sugar and vanilla to firm peaks. Split the cooled choux ring horizontally and spoon the cream over the bottom half.
Add the cooled apples and a generous drizzle of salted caramel over the cream. Replace the top of the ring and drizzle with more caramel before serving.