• 1/2 tsp. cream of tartar
To prepare the crust: Preheat oven to 350 degrees. Lightly coat a pie plate with cooking spray. Stir together cracker crumbs, powdered sugar, butter and milk until well combined. Reserving 1 tablespoon of crumbs for garnish, press the rest into the pie plate, creating an even layer across the bottom and up the sides. Bake for 15 minutes, then let cool on a wire rack.
To prepare the filling: Preheat oven to 375 degrees. Place rhubarb in a shallow baking dish, such as a pie plate, and sprinkle powdered sugar evenly over fruit. Bake for 20 minutes, stirring once, until rhubarb is very soft. (Late summer rhubarb may have less moisture than the first rhubarb of spring, so watch carefully, adding a tablespoon of water if it seems to be scorching before it's soft.)
Carefully place roasted rhubarb in a blender and purée with Triple Sec (or orange juice) and salt. Or whisk briskly in a bowl to create a rough purée. Set aside.
In a saucepan over which a medium bowl will fit, bring about 2 inches of water to a boil. While the water is heating, whisk together egg whites, granulated sugar, and cream of tartar in a medium bowl. Reduce heat to keep the water at a simmer and place the bowl over the saucepan. With a handheld mixer on medium speed, beat the whites until foamy, about 3 minutes. Increase speed to high and beat until the whites begin to appear glossy, moving the beaters around the bowl, about 3 minutes more. Remove bowl from the pan and set on a counter; continue beating mixture until the meringue holds a stiff peak when the beaters are lifted.