Place 2 cups of frozen raspberries, 1 cup of granulated sugar, and 2 tablespoons of lemon juice into a medium saucepan. Heat over medium until bubbling, then reduce to a gentle simmer for 23-25 minutes, stirring occasionally, until it thickens and coats the back of a spoon. Once ready, transfer the jam to a bowl and let it cool in the fridge.

In a small saucepan, combine 1 cup of additional frozen raspberries, 1/2 cup of granulated sugar, and 1/2 cup of water. Bring to a boil over medium-high heat, stirring until the sugar dissolves completely. Once boiling, reduce heat and simmer for 5-7 minutes. Strain the syrup into a bowl to remove solids and let it cool to room temperature while you prepare the mascarpone filling.

In a large mixing bowl, combine 1 cup of mascarpone cheese, 1/2 cup of powdered sugar, 2 tablespoons of lemon juice, and 1 teaspoon of vanilla paste. Beat with an electric mixer on medium speed until creamy and well blended, around 3-4 minutes. In a separate bowl, whip 1 cup of heavy cream until stiff peaks form, then gently fold it into the mascarpone mixture until smooth and fluffy.

Spread a thin layer of the mascarpone filling in the bottom of a 9x13-inch dish. Quickly dip ladyfinger cookies into the raspberry syrup, ensuring they are lightly soaked without becoming soggy. Layer the soaked ladyfingers over the mascarpone filling, then add half of the remaining mascarpone mixture followed by a spoonful of raspberry jam. Repeat this layering process once more, finishing with the mascarpone on top.

Cover the assembled Fresh Raspberry Tiramisu with plastic wrap, pressing it gently to avoid air pockets. Refrigerate it for a minimum of 8 hours or, ideally, overnight.

When ready to serve, carefully remove the tiramisu from the fridge. Spread the remaining raspberry jam on top, then garnish with fresh raspberries and lemon slices for a beautiful presentation. Slice into squares and serve chilled.