Step 1: Make the Raspberry Jam
In a saucepan, combine the raspberries, sugar, and lemon juice. Heat over medium until the raspberries break down and the mixture begins to bubble.

Reduce to a simmer and cook for 23–25 minutes, stirring and mashing occasionally, until thickened. The jam should hold its shape on the back of a spoon.

Transfer to a shallow dish, cover with plastic wrap, and chill until room temperature or cooler.

Step 2: Prepare the Raspberry Syrup
In another saucepan, combine sugar, water, and raspberries. Bring to a boil, then simmer for 3 minutes while breaking down the berries.

Strain the syrup through a fine mesh sieve and discard the seeds. Stir in limoncello (if using), and set aside to cool.

Step 3: Make the Mascarpone Filling
In a large bowl, whisk together mascarpone, powdered sugar, lemon juice, and vanilla until smooth—about 30 seconds.

Add the cold heavy cream and beat until the mixture holds medium-stiff peaks. Be careful not to overmix.

Step 4: Assemble the Tiramisu
Spoon a thin layer of mascarpone cream into the bottom of your baking dish.

Quickly dip each ladyfinger into the raspberry syrup—twice on each side—and place in the dish in a single layer.

Spread half of the mascarpone mixture over the ladyfingers. Smooth the top with a spatula.

Spoon half of the raspberry jam over the mascarpone layer and spread it gently.

Repeat with another layer of dipped ladyfingers and the remaining mascarpone cream.

Cover with plastic wrap and refrigerate for at least 8 hours, or overnight.

Step 5: Finish and Serve
Just before serving, spread the remaining raspberry jam over the top.

Decorate with fresh raspberries and thin slices of lemon if desired.

Slice with a clean knife and serve chilled.