In a medium saucepan, combine frozen raspberries, granulated sugar, and freshly squeezed lemon juice. Cook over medium heat for about 23-25 minutes, stirring occasionally until the mixture thickens and resembles a jam. Once done, remove it from heat and let it cool.

Refrigerate the jam to help it firm up while you prepare the other components.

In a small saucepan, combine water, a handful of raspberries, and sugar, bringing the mixture to a boil over medium heat. Reduce the heat and simmer for approximately 3 minutes, allowing the raspberries to soften. Strain the syrup through a fine-mesh sieve and set it aside to cool.

In a mixing bowl, beat the mascarpone cheese, powdered sugar, lemon juice, and vanilla paste together until smooth and creamy.

Gradually add in the heavy cream, continuing to beat until medium-stiff peaks form, which should take about 2-4 minutes using an electric mixer.

Begin layering your Raspberry Tiramisu by briefly dipping ladyfinger cookies in the raspberry syrup.

In a rectangular dish, spread a layer of the mascarpone filling, followed by a layer of the syrup-soaked ladyfingers. Repeat this process until all ingredients are used, finishing with a layer of mascarpone.

Cover the assembled dessert with plastic wrap and refrigerate it for a minimum of 8 hours, allowing the flavors to meld and the layers to set perfectly.

Once set, remove the tiramisu from the refrigerator and spread the remaining raspberry jam on top. Garnish with fresh raspberries and lemon slices for an added touch of vibrancy.

Slice into squares and serve chilled, allowing your guests to indulge in every luscious bite of this refreshing Raspberry Tiramisu.