Make the raspberry jam: Heat frozen raspberries, sugar, and lemon juice in a saucepan. Simmer for 25 minutes, mashing the raspberries periodically, until thickened. Let cool completely in the fridge.
Prepare raspberry syrup: Cook sugar, water, and raspberries in a small saucepan until the sugar dissolves. Simmer for 3 minutes, then strain and mix with limoncello. Cool to room temperature.
Whip the mascarpone filling: Combine mascarpone, powdered sugar, lemon juice, and vanilla paste. Add heavy cream and whip to medium-stiff peaks.
Assemble the tiramisu: In a baking dish, spread a small amount of mascarpone cream. Dip ladyfingers into the syrup and layer them in the dish. Alternate with layers of cream, raspberry jam, and more ladyfingers.
Chill and serve: Cover and refrigerate for at least 8 hours, ideally overnight. Add a final layer of raspberry jam before serving, and garnish with fresh raspberries and lemon slices.