In a medium saucepan, combine frozen raspberries, granulated sugar, and lemon juice. Cook over medium heat for 23-25 minutes, stirring occasionally, until the mixture thickens and bubbles. Remove from heat and cool completely.

In a small saucepan, combine sugar, water, and frozen raspberries. Bring to a simmer over medium heat for about 3 minutes, stirring until sugar dissolves. Strain syrup and let it cool.

In a mixing bowl, beat together mascarpone cheese, powdered sugar, lemon juice, and vanilla paste until smooth. In a separate bowl, whip heavy cream to medium-stiff peaks and fold into mascarpone mixture.

Dip ladyfingers briefly in the cooled raspberry syrup and layer half in the bottom of a rectangular baking dish. Spread half of the mascarpone filling over the layer. Repeat with remaining ingredients, finishing with mascarpone on top.

Cover with plastic wrap and refrigerate for at least 8 hours, preferably overnight.

After chilling, remove plastic wrap, top with remaining raspberry jam, and garnish with fresh raspberries and lemon slices. Serve chilled.