Lightly butter a 9” x 12” (or roughly 2 quart) ceramic baking dish.
Chop ends off the rhubarb and then chop into ½” – ¾” thick pieces—this way some pieces almost melt, while some retain their shape, but that said if you want more of a whole “fruit” feel that you might get with an apple crumble cut the rhubarb into 1 ½” – 2” chunks (but you may want to bake the crumble a little longer). Anyway, add these neon chunks to a large mixing bowl and mix with all the other filling ingredients until well combined. Transfer to the baking dish and set aside.
For the crumble topping, either with a food processor or with your hands, mix the flour, meal, sugar and salt. Break the butter up into fine pieces. If you’re using your hands do so quickly, rubbing the mixture between your fingers and the pads of your thumbs. The mixture should resemble damp sand.
Stir in the oats and slivered almonds.
Top the rhubarb mixture with evenly with the crumble. You can squeeze some of the crumble in your fist and break coarsely over the rhubarb for large pieces of crumble topping.
Bake for 35-40 minutes until the crumble is slightly browned and the juices in the rhubarb are thick and bubbly. Allow this to cool for 20 minutes or so before serving with ice cream; French vanilla, strawberry or even caramel all are fantastic as an accompaniment to this.
Vanilla Sugar is basically made by filling a sterilized jar halfway with sugar, plunking in a vanilla pod (whole, or the carcass of one that you've split open and scraped the seeds from), and filling with more sugar. Leave it to sit, untouched, for about 2 weeks, then shake and give a few more days before using. As you use the vanilla-scented sugar, top the jar off with more regular white sugar--it only needs a day to be infused with vanilla flavor once you have a base going. If you don't have vanilla sugar, though, just add another 1 to 1-1/2 t-spoons of vanilla extract to the mix.