Have a 13″ x 9″ glass dish ready (or two smaller 8″ x 8″ or 9″ x 9″ pans.
Glass or ceramic work best because you have to scrape the granita with a fork, and it could ruin the finish of a metal pan).
Wash the rhubarb, and cut it into thin slices until you have about 3 cups total.
Put the rhubarb slices, blueberries, water and honey in a medium saucepan.
Cook over medium heat for about 10 minutes, stirring often.
Be careful because the mixture tends to bubble up. If that happens, turn the heat down a bit.
Pour the fruit mixture into a fine mesh strainer over the 13″ x 9″ dish.
Use a spoon to mash and stir the mixture through the strainer, catching the liquid underneath.
Work it until all that remains inside the strainer is a thick pulp.
Discard the pulp.
You’ll have to babysit the granita a bit from here.
Place the dish in the freezer and allow to harden for about 20-30 minutes.
Then, scrape the dish down with a fork. Your goal is to create small icy chunks.
I checked on mine about every 30 minutes after that, scraping it down further. Eventually it’ll get firm enough to where you can scrape right across the top.
*For a naughty mix up you could add 'alcohol' instead of water for a yummy frozen slushie.