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For the rhubarb syrup:
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In a medium saucepan, add the chopped rhubarb, sugar, water and vanilla bean and seeds. Cook over medium heat, until the rhubarb is very mushy, about 20 minutes.
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Set a fine mesh sieve over a heat-proof bowl and drain the contents of the pot through the sieve. Rinse any bits out of the pot if that is the pot you plan to use to make the rest of the mousse. No need to wash it.
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Measure the liquid into a glass measuring cup and discard the mushy rhubarb and vanilla bean. You should have one cup of syrup. If you have less, add a smidge of water to top it off. If you have more, then just use one cup and save the excess for a cocktail or to add to some sparkling water.
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For the rhubarb mousse:
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Sprinkle the gelatin powder over the lemon juice in a small bowl and set it aside.
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In a small bowl, lightly whisk the egg yolks with half of the sugar while slowly adding some of the warm rhubarb syrup to temper the eggs. Then add the tempered egg yolks into a medium saucepan with the rest of the rhubarb syrup.
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Over a medium low heat, whisk the rhubarb mixture constantly until it starts to thicken. This should take about ten minutes but pay close attention to it and don't let it boil. It will change from a splashy liquid, to going quite frothy. When it starts to get frothy, that means that it is close to being done. Remove it from the heat and let the froth die down a bit and check to see if the rhubarb mixture is done by coating the back of a wooden spoon with it and running your finger through it. If your finger mark remains, it is ready. If the liquid starts to run back together, keep cooking it for a little while longer.
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Off the heat, whisk in the bloomed gelatin. Pour the rhubarb mixture into a medium metal bowl, preferably one that sits well inside another larger bowl.
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Either place the rhubarb mixture in the fridge for 25-30 minutes and check it frequently to monitor how it is thickening and to stir it often. When it feels quite cool to the touch and it has visibly thickened to the point that the lines made when you stir it don't disappear right away, you can take it out of the fridge. The faster way to cool it down is to put lots of ice and water in a larger bowl and set the bowl with the rhubarb mixture in it and whisk constantly for about 5 minutes. It will cool down quickly and start to thicken much faster. Once it starts to thicken and is cold to the touch, take the rhubarb mixture bowl out of the ice water bowl. Set it aside. If it continues to get thick, just whisk it vigorously before you add the beaten egg whites.
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In a clean bowl, whisk the egg whites using a stand mixer or electric beaters until very frothy and then slowly start adding the other half of the sugar as you continue to whisk. Keep beating the egg whites until you get stiff peaks. Take 1/4 of the stiff egg whites and stir it into the cold, thickened rhubarb mixture to lighten it up a little bit. Once the first bit of egg whites is incorporated, add the rest of the egg whites and gently fold the egg whites in using a rubber spatula until there are no streaks of the darker rhubarb mixture.
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Once the two parts are incorporated, spoon the mousse into 6 dessert bowls or champagne glasses and refrigerate for at least 2 hours before serving. Serve with a dollop of lightly sweetened whipped cream if desired.
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Note: For those with sensitivities, this does include raw eggs. Use pasteurized egg whites if serving this to anyone who shouldn't eat raw egg whites. Save the four egg whites and freeze them to be used in a recipe that calls for cooked egg whites, such as a pavlova.
