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 bowl, pressing to extract all liquid. Discard seeds and pulp. Return the strained liquid to the saucepan.
03 - Sprinkle the 2 envelopes of gelatin 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. Remove from heat.
05 - Pour the berry gelatin mixture into serving glasses or a trifle dish, filling about halfway. Refrigerate for at least 2 hours until completely set and firm to the touch.
06 - Scrape vanilla bean seeds into a saucepan and add the milk (or add vanilla extract to milk). Heat just until steaming, stirring occasionally. Remove from heat before boiling.
07 - Whisk egg yolks and 1/3 cup 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. Cook over low heat, stirring constantly, until thickened enough to coat the back of a spoon. Do not boil or eggs will scramble.
09 - Sprinkle 1 1/2 teaspoons gelatin over 1 tablespoon cold water. Let bloom 5 minutes, then stir into the warm custard until dissolved. Cool to room temperature, stirring occasionally.
10 - Using an electric mixer, whip the cold heavy cream until soft peaks form. Lift the whisk to check that cream holds its shape but still has soft curling tips.
11 - Gently fold one-third of the whipped cream into the cooled custard to lighten it. Add remaining cream and fold until completely incorporated and smooth.
12 - Spoon or pipe the vanilla mousse over the set berry jello layer in each serving glass. Smooth the top with an offset spatula.
13 - Refrigerate the assembled desserts for at least 2 more hours until the mousse layer is firm and completely set.
14 - Top each serving with fresh berries and mint leaves just before serving. Serve chilled.