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 reduces.

In a small saucepan, combine water and granulated sugar, stirring until dissolved. Add remaining raspberries and heat the mixture over low to medium heat, simmering for about 3 minutes. Strain the syrup through a fine mesh sieve to remove solids, letting it cool completely.

In a large mixing bowl, blend cold mascarpone cheese, powdered sugar, lemon juice, and vanilla paste using an electric mixer on medium speed until smooth. Gradually whisk in heavy cream until the mixture reaches medium-stiff peaks.

Layer soaked ladyfinger cookies in a 9x9 baking dish, followed by half of the mascarpone mixture. Repeat this process with another layer of ladyfingers and the remaining mascarpone filling, smoothing the top as you go.

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

Spread the remaining raspberry jam on top and decorate with fresh raspberries and lemon slices before serving.