In a medium saucepan, combine frozen raspberries, granulated sugar, and lemon juice over medium heat. Stir occasionally and bring to a bubbling simmer. Cook for about 23-25 minutes, or until it thickens slightly and the raspberries break down.

Once the raspberry jam has thickened, carefully transfer it to a shallow bowl. Allow it to cool to room temperature for about 30 minutes, then refrigerate for at least 1 hour.

In another saucepan, combine sugar, water, and a handful of raspberries over medium heat. Allow it to simmer for 3 minutes, stirring to dissolve the sugar completely. Strain the syrup through a fine-mesh sieve to remove the solids.

In a large mixing bowl, whisk together the mascarpone cheese, powdered sugar, lemon juice, and vanilla paste until smooth and creamy. In a separate bowl, whip the heavy cream until medium-stiff peaks form.

Gently fold the whipped cream into the mascarpone mixture, ensuring a light and airy consistency.

Spread a generous layer of the mascarpone filling at the bottom of a baking dish. Dip the ladyfinger cookies into the raspberry syrup briefly and layer them over the mascarpone, followed by another layer of mascarpone filling and half of the cooled raspberry jam.

Cover the assembled Fresh Raspberry Tiramisu with plastic wrap and refrigerate for at least 8 hours, or overnight if possible.

Before serving, spread the remaining raspberry jam over the top layer. Garnish with fresh raspberries and lemon slices for added visual appeal.