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In a small bowl, sprinkle the gelatin over the water and let stand until softened, about 5 minutes. In a blender, puree 4 cups of the raspberries with 1/2 cup of the sugar. Strain the puree into a bowl through a fine sieve.
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Heat the gelatin in a microwave for 10 seconds or so on low power. Whisk the gelatin into the raspberry puree.
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In a large bowl set over a pot of simmering water, whisk the egg whites with the remaining 1/4 of cup sugar until warm to the touch. Remove the bowl from the heat, and using an electric mixer, beat the whites at medium-high speed until stiff and glossy. Fold the egg whites into the raspberry puree.
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Beat the cream until firm. Fold whipped cream into the raspberry mixture. Cover and refrigerate until set, at least 1 hour or overnight.
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Stir together rhubarb and sugar in a large saucepan and let sit until rhubarb releases some of its liquid, about 10 minutes.
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Bring rhubarb mixture to a boil over medium-high heat, stirring occasionally. Reduce heat and simmer, stirring occasionally, until rhubarb has softened but some whole pieces remain, about 5 minutes. Remove from heat and stir grated ginger into rhubarb mixture. Allow to cool completely.
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Bring rhubarb mixture to a boil over medium-high heat, stirring occasionally. Reduce heat and simmer, stirring occasionally, until rhubarb has softened but some whole pieces remain, about 5 minutes. Stir in ginger and allow to cool completely.
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To assemble, divide one half of raspberry mousse evenly among clear glasses or bowls. Top with rhubarb compote and finish with remaining raspberry mousse. Garnish each with a few fresh raspberries.