For the Meringues: Preheat oven to 200 F and cover several baking sheets with parchment paper.
In an electric stand mixer fitted with the whisk attachment add the egg whites, cream of tartar and salt. Make sure the bowl and whisk are freshly cleaned and dried. Whisk on medium until the eggs are frothy and then slowly add in the granulated sugar. This should take several minutes. Turn the mixer on high and when the meringue is shiny and glossy and almost to stiff peaks turn off the mixer and add in the powdered sugar and vanilla. Turn the mixer on high until you reach stiff peaks, should be just 1 minute more. The meringue will look glossy and hold its shape when you invert the whisk. The slope of the invert meringue should be at or almost at 12 o'clock.
Use two spoons to dollop nine, 4-inch circles of meringue onto the baking sheet(s), leaving about two inches between each meringue to allow for heat circulation. Use the back on your spoon to make a large valley in the center of the meringue, this is where you will later put the whipped cream and berries. You can use the spoons to smooth out the sides of the meringues. They will look exactly like they do right now when they come out of the oven, so make sure you're dazzled.
Bake one sheet at a time in the center of the oven.* The meringues will need to be baked for at least 2 hours or all the way up to 3 hours. Check after 2 hours to make sure that the meringues aren’t taking on any color. When done, they will feel crisp on the outside and if you cut into one they will be cooked through. Turn off the oven and let the meringues sit in there until completely cool; this could take several hours.
Once completely cool and dried, store in an airtight container at room temperature for several days...they often last longer. Separate layers between parchment paper. *If you can't get all of the meringues on one baking sheet you could bake two sheets at a time, rotating front to back and top to bottom midway through the bake time. I haven't baked two sheets at a time, as I use an extra-large baking sheet. So if you do, please let us know how it goes and take extra care to make sure they don't brown too quickly.
For the Roasted Rhubarb: Preheat your oven to 350 F and covered a rimmed baking sheet in parchment.
Add the rhubarb, berries, brown sugar, granulated sugar, balsamic and cinnamon and toss to coat. Get in there with your hands and make it happen.
Roast in the center of the oven for 30 minutes. Set aside to cool completely.
For the Whipped Cream: In an electric stand mixer fitted with the whisk attachment whisk the whipping cream on medium high until the peaks are just starting to get firm and stiff...I don't like my whipped cream too stiff. Take care not to over whip your cream as you'll start to enter churned butter territory.
To Assemble: Place your meringues on a dessert plate, dollop with some whipped cream, sprinkle with your slivered almonds, drizzle with the roasted fruit (making sure to spoon over some of the juice) and then sprinkle with more almonds. Don't drizzle with the fruit until ready to eat because it will stain the whipped cream and look funky if it sits there too long.