Preparation

Make the flavor base:

• In a food processor, combine the raspberries, blackberries, 1/4 cup of the sugar, the lemon juice, and salt. Purée until smooth. Pass the purée through a fine sieve; discard the contents of the sieve. Set the purée aside.
• Choose a medium stainless-steel bowl that can rest just inside a medium saucepan. Pour about 1 inch of water in the saucepan and bring to a boil. Choose a large bowl that’s big enough for the medium bowl to fit inside it. Fill the large bowl with ice water; set aside.
• Put the egg yolks, 1/4 cup of the sugar, and 1/2 cup of the berry purée in the medium bowl. When the water in the saucepan boils, reduce the heat to a gentle simmer, and set the bowl with the berry mixture on the saucepan. Whisk until the mixture reaches 140°F on an instantread thermometer. Turn off the heat but leave the bowl over the water and continue whisking for 31/2 minutes. (If the temperature reaches 150°F, remove the bowl from the water, whisk until the temperature drops to 145°F, and then return the bowl to the water bath to continue whisking.) Remove the bowl from the saucepan, stir in the remaining berry purée, and set the medium bowl into the ice bath to cool. Don’t pour the hot water out of the saucepan.

Bloom and dissolve the gelatin:

• Pour 1/4 cup cold water into a small heatproof custard cup; sprinkle the gelatin evenly over the water in the cup. Let sit for about 5 minutes. Set the custard cup with the gelatin into the saucepan with the hot water and stir the gelatin mixture until the gelatin melts and becomes translucent, about 2 minutes.
• Once the gelatin has melted, whisk it into the berry mixture. Whisk occasionally until the mixture cools to 50° to 55°F and thickens slightly.

Beat the egg whites and cream:

• In a dry, grease-free bowl, beat the egg whites with a hand mixer (be sure the beaters are dry and free of grease, too) at low speed until frothy; add the cream of tartar (it helps stabilize the foam). Increase the speed to medium high. Beat until the whites turn opaque, begin to thicken, and look foamy, 1-1/2 minutes. Gradually beat in the remaining 1/4 cup sugar. Continue beating until the whites look thick and shiny (but not dry), resemble thickly whipped cream, and form medium peaks, about 3 minutes.

  

• In a separate bowl (no need to clean the beaters this time), beat the cream until soft peaks form, about 2 minutes.

Assemble the mousse:

• Whisk a couple of large spoonfuls of the beaten egg whites into the berry mixture to lighten it. Pour the ice and water out of the ice bath bowl; dry the bowl. Pour the lightened berry mixture into the cold bowl.

  

• Scrape the remaining egg whites and cream on top and fold gently with a rubber spatula until just combined. Spoon the mousse into individual glasses, into a 6-cup bowl, or into individual ramekins. Refrigerate, covered, until firm, at least 4 hours but no longer than 24 hours. Garnish with reserved whole berries and sprigs of mint, if you like.