Add the raspberries, sugar, and lemon juice to a saucepan and heat until the raspberries are no longer frozen, have started to break down, and the resulting liquid has come to a bubble.

Once bubbling, turn it down to a simmer. Stir occasionally and mash down raspberries with a rubber spatula. Let it simmer for 23-25 minutes or until the jam thickens.

Pour the jam into a shallow bowl to let it cool down faster, and place it in the fridge, covered with plastic wrap. Let it cool down until room temperature or colder, about 1 hour.

While the jam cools, place the sugar, water, and raspberries in a small saucepan over high heat until the sugar dissolves and starts to boil.

Turn down the heat to a simmer and let it simmer for 3 minutes. Stir occasionally and break down the berries.

Strain the syrup into a bowl using a fine mesh sieve and discard the raspberry kernels. Pour in the limoncello and let it cool down to room temperature.

Once the raspberry jam and syrup are cool, make the filling. Cream the mascarpone, powdered sugar, lemon juice, and vanilla paste using a mixer until combined, about 30 seconds.

Add the heavy cream to the mascarpone mixture and whisk until it can hold a medium-stiff peak.

Use a 27x20 cm (8x10.5 inch) or 23x23 cm (9x9 inch) baking dish. Start by adding a bit of mascarpone cream to the bottom and evening it out.

Dip the lady fingers into the raspberry syrup twice on each side. Layer the lady fingers in the baking dish, add half of the mascarpone cream and half of the raspberry jam.

Repeat the process but for the raspberry jam, that will be added once the tiramisu is ready to serve.

Cover with plastic wrap and let it set in the fridge for a minimum of 8 hours, preferably overnight.

Once ready to serve, add the remaining raspberry jam and spread it in an even layer. Optionally, decorate with fresh raspberries and lemon slices.