In a medium saucepan over medium heat, combine frozen raspberries, granulated sugar, and lemon juice. Cook until it bubbles, about 5 minutes, then simmer for 23-25 minutes until thickened. Cool completely before use.

In another saucepan, mix water, raspberries, and sugar over high heat until the sugar dissolves. Simmer for 3 minutes, strain through a sieve. Cool before use.

In a large mixing bowl, combine mascarpone, powdered sugar, lemon juice, and vanilla. Blend until smooth. Whip heavy cream to medium-stiff peaks and fold into the mascarpone mixture.

In a serving dish, layer mascarpone filling at the bottom. Dip ladyfingers quickly into raspberry syrup and lay on top, repeating the layers. Cover and refrigerate for at least 8 hours.

Remove from refrigerator, spread remaining raspberry jam on top, garnish with fresh raspberries and lemon slices. Slice and serve chilled.