In a medium saucepan, combine frozen raspberries, granulated sugar, and lemon juice over medium heat. Stir occasionally until the mixture comes to a gentle boil, about 5 minutes. Reduce the heat to low and let it simmer for 23-25 minutes until thickened, stirring to prevent sticking.

Pour the raspberry jam into a bowl and let it cool for a few minutes at room temperature. Cover with plastic wrap and refrigerate until completely cooled, about 30 minutes.

In another saucepan, combine water, granulated sugar, and frozen raspberries over medium heat. Stir until the sugar dissolves, then bring to a boil for approximately 2 minutes. Reduce heat to low and simmer for another 3 minutes. Strain the mixture through a fine-mesh sieve.

In a mixing bowl, combine mascarpone cheese, powdered sugar, lemon juice, and vanilla paste. Beat with an electric mixer until smooth and well-aerated, about 2-3 minutes. In a separate bowl, whip heavy cream until it forms medium-stiff peaks.

Spoon a small dollop of the mascarpone mixture into the bottom of a 9x9-inch baking dish. Dip ladyfinger cookies into the raspberry syrup for 2-3 seconds. Layer half of the dipped ladyfingers over the mascarpone, then add half of the remaining mascarpone mixture and half of the cooled raspberry jam. Repeat the layers.

Cover the assembled dessert with plastic wrap and refrigerate for at least 8 hours, preferably overnight.

When ready to serve, remove the plastic wrap and top with the remaining raspberry jam. Garnish with fresh raspberries and lemon slices if desired.