In a medium saucepan, combine frozen raspberries, granulated sugar, and freshly squeezed lemon juice. Cook over medium heat until the mixture begins to bubble, then reduce the heat and let it simmer for 23-25 minutes, stirring occasionally, until it thickens. Transfer to a bowl and chill in the fridge to cool completely.

In another saucepan, combine sugar and water over high heat, stirring until the sugar dissolves. Add a cup of raspberries and simmer for about 3 minutes. Strain the mixture through a fine sieve to remove solids and let the syrup cool.

In a large mixing bowl, blend mascarpone cheese, powdered sugar, lemon juice, and vanilla paste until smooth. Gradually add heavy cream and whip until medium-stiff peaks form.

Spread a thin layer of mascarpone filling at the bottom of a baking dish. Quickly dip ladyfinger cookies in the raspberry syrup and layer over the filling. Alternate with half of the mascarpone mixture, followed by half of the raspberry jam, repeating layers until all ingredients are used.

Cover the baking dish with plastic wrap and refrigerate for at least 8 hours. Overnight chilling is ideal for developing flavors.

Before serving, remove the tiramisu from the fridge and top with the remaining raspberry jam. Garnish with fresh raspberries and lemon slices.