01 - Combine mixed berries, sugar, and 1 cup water in a saucepan. Bring to a simmer and cook for 5 minutes, gently mashing berries with a spoon to release juices.
02 - Pour the cooked berry mixture through a fine mesh sieve into a clean container, pressing firmly to extract all liquid. Discard seeds and pulp, then return the strained liquid to the saucepan.
03 - Sprinkle gelatin powder over 1/2 cup cold water in a small bowl. Let stand for 5 minutes until the gelatin absorbs the water and becomes spongy.
04 - Stir the bloomed gelatin into the hot berry liquid until completely dissolved. Add lemon juice and mix thoroughly.
05 - Pour the berry mixture into the bottom of serving glasses or a trifle dish, filling about halfway. Refrigerate for at least 2 hours until completely firm.
06 - Split the vanilla bean lengthwise and scrape seeds into a saucepan. Add milk and heat just until steaming, then remove from heat.
07 - Whisk egg yolks with sugar in a bowl until pale and thick. Slowly pour the hot vanilla milk into the yolks while whisking constantly to prevent curdling.
08 - Return the mixture to the saucepan and cook over low heat, stirring continuously, until slightly thickened and coating the back of a spoon. Do not let the mixture boil.
09 - Sprinkle gelatin over 1 tablespoon cold water and let bloom for 5 minutes. Stir the bloomed gelatin into the warm custard until fully dissolved.
10 - Let the custard cool to room temperature, stirring occasionally to prevent a skin from forming on the surface.
11 - Beat cold heavy cream with an electric mixer until soft peaks form when the whisk is lifted.
12 - Gently fold the whipped cream into the cooled custard in three additions, being careful not to deflate the mixture.
13 - Once the berry jello is firmly set, carefully spoon or pipe the vanilla mousse over the jello layer in each serving glass or dish.
14 - Refrigerate the assembled desserts for at least 2 more hours until the mousse layer is completely set.
15 - Top each dessert with fresh berries and mint leaves just before serving.