In a medium saucepan over medium heat, combine frozen raspberries, granulated sugar, and lemon juice. Bring to a boil, then reduce heat and simmer for 23-25 minutes until thickened. Cool and refrigerate.

In a small saucepan, add water, sugar, and frozen raspberries. Simmer, stirring until sugar dissolves. Strain to remove seeds and cool completely.

In a large bowl, beat mascarpone cheese, powdered sugar, lemon juice, and vanilla paste until smooth. Gradually add heavy cream and whisk until medium-stiff peaks form.

Spread a layer of mascarpone filling in a baking dish. Dip ladyfingers in raspberry syrup and arrange them in a layer over the filling. Repeat layering with remaining ingredients.

Cover and refrigerate for at least 8 hours. This allows flavors to meld and the dessert to set.

Before serving, top with raspberry jam and optionally garnish with fresh raspberries and lemon slices.