In a saucepan, combine frozen raspberries, 100g sugar, and 1 tablespoon of lemon juice over medium heat. Bring to a gentle bubble, then reduce heat and simmer for 23-25 minutes until thickened.

In another saucepan, mix 100g of sugar with 120g of water and 30g of raspberries over medium heat. Boil, then lower heat and simmer for about 3 minutes. Strain the syrup to remove raspberry solids and cool completely.

In a large mixing bowl, add 450g of mascarpone cheese, 120g of powdered sugar, and 2 tablespoons of lemon juice. Beat until smooth. Gradually incorporate 480g of chilled heavy cream until medium-stiff peaks form.

In a baking dish, spread a layer of whipped mascarpone filling, dip ladyfinger cookies in the raspberry syrup, and arrange over the mascarpone. Repeat layering with more mascarpone and raspberry jam, ending with mascarpone.

Cover with plastic wrap and refrigerate for at least 8 hours, ideally overnight. Serve chilled, garnished with remaining raspberry jam, fresh raspberries, and lemon slices.