Make the Raspberry Jam:

1. Combine raspberries, sugar, and lemon juice in a saucepan. Heat until raspberries break down and begin to bubble.

2. Simmer for 23–25 minutes, mashing occasionally until thickened. Test by drawing a line through the jam on the back of a spoon—if it holds, it’s ready.

3. Cool in a shallow bowl in the fridge, covered.

Make the Raspberry Syrup:

1. Boil sugar, water, and raspberries in a small saucepan until sugar dissolves. Simmer for 3 minutes, mashing berries.

2. Strain and discard solids. Stir in limoncello. Cool to room temperature.

Prepare the Mascarpone Filling:

1. Beat mascarpone, powdered sugar, lemon juice, and vanilla until just combined.

2. Add heavy cream and whip to medium-stiff peaks.

Assemble the Tiramisu:

1. Spread a thin layer of mascarpone filling in the base of a 27×20 cm or 23×23 cm dish.

2. Dip ladyfingers twice in raspberry syrup and arrange in a layer.

3. Spread half the mascarpone mixture, then half the raspberry jam. Repeat with another layer of ladyfingers and mascarpone (reserve the rest of the jam for topping).

4. Cover and chill for at least 8 hours or overnight.

To Serve:
Top with the remaining raspberry jam, and decorate with fresh raspberries and lemon slices.