Make Raspberry Jam: In a medium saucepan, combine frozen raspberries, granulated sugar, and lemon juice. Heat over medium-high until bubbling, then reduce to a simmer for 23-25 minutes, stirring occasionally until thickened. Remove from heat and let cool in the fridge.

Prepare Raspberry Syrup: In another saucepan, combine sugar, water, and raspberries over medium heat. Bring to a boil, stirring until sugar dissolves completely. Strain through a sieve into a bowl, discarding solids. Cool to room temperature.

Make Mascarpone Filling: In a mixing bowl, blend mascarpone cheese, powdered sugar, lemon juice, and vanilla paste until smooth. In another bowl, whip cold heavy cream until medium-stiff peaks form. Fold whipped cream into mascarpone mixture in three additions.

Assemble Tiramisu: Spread a layer of mascarpone filling in a dish. Dip ladyfinger cookies in raspberry syrup and layer in the dish. Layer half of the mascarpone mixture and raspberry jam, repeat and finish with mascarpone.

Chill and Set: Cover with plastic wrap and refrigerate for at least 8 hours. This allows flavors to blend seamlessly.

Serve: Remove from fridge, spread remaining raspberry jam on top, garnish with fresh raspberries and lemon slices. Slice and enjoy!