In a medium saucepan, combine 2 cups of frozen raspberries with 1 cup of granulated sugar and 2 tablespoons of freshly squeezed lemon juice. Heat over medium heat until bubbling, then reduce to low and simmer for 23-25 minutes, stirring occasionally, until thickened. Remove from heat, allow to cool, then refrigerate until completely chilled.

In a separate saucepan, bring 1 cup of sugar and 1 cup of water to a boil over medium-high heat. Once boiling, add the remaining raspberries, stirring to dissolve the sugar. Cook for an additional 5 minutes, then strain the mixture to remove solids. Let the raspberry syrup cool completely before using.

In a large mixing bowl, beat together 1 cup of mascarpone cheese, 1/2 cup of powdered sugar, 1 tablespoon of lemon juice, and 1 teaspoon of vanilla paste until smooth and creamy. In a separate bowl, whip 1 cup of heavy cream until medium-stiff peaks form. Gently fold the whipped cream into the mascarpone mixture, ensuring a light and airy texture.

In a rectangular dish, quickly dip ladyfinger cookies into the prepared raspberry syrup, ensuring they're soft but not soggy. Layer half of the mascarpone filling over the soaked ladyfingers, then spread half of the refrigerated raspberry jam on top. Repeat with another layer of dipped ladyfingers, remaining mascarpone filling, and the rest of the raspberry jam.

Cover the assembled Fresh Raspberry Tiramisu with plastic wrap and refrigerate for at least 8 hours, or ideally overnight. This chilling time allows the flavors to meld beautifully.

Before serving, drizzle any remaining raspberry jam over the top of the dessert for added flavor. Garnish with fresh raspberries and decorative lemon slices to enhance the visual appeal.