Raspberry Jam
In a saucepan, combine raspberries, sugar, and lemon juice. Heat until berries thaw and begin to break down.
Bring to a boil, then reduce to a simmer. Cook for 23–25 minutes, stirring and mashing occasionally, until thickened.
Transfer to a shallow bowl, cover, and chill until cooled (about 1 hour).
Raspberry Syrup
In a saucepan, combine sugar, water, and raspberries. Heat until boiling and sugar dissolves.
Reduce heat and simmer for 3 minutes, mashing berries.
Strain through a sieve, discard seeds, and stir in limoncello (optional). Let cool.
Mascarpone Filling
Beat mascarpone, powdered sugar, lemon juice, and vanilla until smooth (about 30 seconds).
Add heavy cream and whip until medium-stiff peaks form.
Assemble the Tiramisu
Spread a thin layer of mascarpone cream on the bottom of your dish.
Dip ladyfingers briefly in raspberry syrup (both sides) and layer them in the dish.
Spread half the mascarpone cream over the layer, then add half the raspberry jam.
Repeat layers (ladyfingers + mascarpone cream).
Cover and refrigerate for at least 8 hours or overnight.
Finish & Serve
Before serving, spread the remaining raspberry jam on top.
Garnish with fresh raspberries and lemon slices. Slice and enjoy chilled.