In a medium saucepan over medium heat, combine frozen raspberries, granulated sugar, and fresh lemon juice. Stir occasionally until bubbling, then reduce heat and simmer for 23-25 minutes until thickened. Cool in the refrigerator.

In a small pot, combine sugar, water, and frozen raspberries. Heat until sugar dissolves, then gently simmer for 5-7 minutes. Strain through a fine-mesh sieve and cool to room temperature.

In a large mixing bowl, combine mascarpone cheese, powdered sugar, fresh lemon juice, and vanilla paste. Beat until smooth; gradually add heavy cream while whipping until medium-stiff peaks form.

Dip ladyfinger cookies in the raspberry syrup and layer half of the mascarpone filling in a baking dish, followed by ladyfingers and half of the raspberry jam. Repeat and finish with remaining mascarpone filling.

Cover with plastic wrap and refrigerate for at least 8 hours, preferably overnight, to let the flavors meld.

Before serving, spread remaining raspberry jam on top, garnish with fresh raspberries and lemon slices, cut into squares, and serve chilled.