In a medium saucepan, combine frozen raspberries, granulated sugar, and lemon juice over medium heat. Stir occasionally until the mixture reaches a bubble, then reduce heat to low, simmering for 23-25 minutes. Once ready, pour the jam into a shallow bowl and allow it to cool completely.

In a small saucepan, combine water, sugar, and the remaining raspberries over medium heat. Stir until the sugar dissolves, then let it simmer for about 3 minutes. Once the syrup thickens slightly, strain the mixture through a fine sieve to remove seeds, leaving you with a smooth raspberry syrup.

In a mixing bowl, beat together mascarpone cheese, powdered sugar, lemon juice, and vanilla paste using an electric mixer on medium speed until smooth. In a separate bowl, whip the heavy cream to medium-stiff peaks, then gently fold it into the mascarpone mixture.

In a 9x9-inch dish, layer by soaking ladyfinger cookies in raspberry syrup, then arranging them in the dish. Spread a layer of mascarpone filling over the ladyfingers, followed by a layer of raspberry jam. Repeat until all components are used, finishing with mascarpone on top.

Cover the assembled Raspberry Tiramisu and refrigerate for at least 8 hours or overnight. This allows the flavors to meld together beautifully.

Just before serving, remove the Raspberry Tiramisu from the refrigerator, spread remaining raspberry jam on top and garnish with fresh raspberries and lemon slices. Slice into squares and serve chilled.