In a medium saucepan, combine frozen raspberries, granulated sugar, and lemon juice. Cook over medium heat until bubbling, then simmer until thickened for about 23–25 minutes.

In another saucepan, combine sugar, water, and raspberries. Bring to a boil, then lower the heat and simmer for about 3 minutes. Strain through a fine sieve and cool, optionally adding limoncello.

In a large bowl, beat mascarpone, powdered sugar, lemon juice, and vanilla paste until smooth. Add heavy cream and whip until medium-stiff peaks form, about 3–4 minutes.

In a baking dish, spread a layer of mascarpone, dip ladyfingers in raspberry syrup, layer them on top, then repeat with mascarpone and raspberry jam, finishing with mascarpone.

Cover and refrigerate for at least 8 hours or overnight for flavors to meld.

Before serving, garnish with fresh raspberries and lemon slices, slice, and enjoy.