Cut a piece of foil or parchment longer than the circumference of a 6-cup/1.5-liter mold. Double it for stiffness. Wrap around the outside rim so that it rises at least an inch above the top of the dish, and tape to secure.
Puree the berries with 1/4 cup/55 g sugar, the lemon juice and salt. Strain and reserve.
Put the yolks in a nonreactive bowl with 1/4 cup/55 g sugar and 1/2 cup/125 g of the berry puree. (Reserve the remaining puree for another use.) Set the bowl over a saucepan of barely simmering water, making sure the bowl does not touch the water below, and whisk until the sugar dissolves and it is warm in the center. Remove the bowl from the saucepan and set aside.
Put 1/4 cup/60 ml water into a ramekin, sprinkle the gelatin on top and let soften 5 minutes. Set the ramekin in the hot water in the saucepan (make sure the water does not come up over the edge of the ramekin) and let it melt, about 2 minutes. It should be translucent. Whisk this into the berry mixture, and let cool until it thickens slightly. Don't let it set too far.
Beat the whites with the cream of tartar until foamy. Gradually incorporate the remaining 1/4 cup/55 g sugar, and beat to thick, shiny peaks like whipped cream. Whip the cream in a separate bowl. Stir a spoonful of the whites into the berry mixture to lighten it. Scrape the remaining whites and cream on top and fold together.
Spoon into the prepared mold, cover and refrigerate until firm, about 4 hours.
To serve, remove the collar and spoon into dishes. This is best eaten on the day it is made or the next day. After that, the gelatin starts to break down.