In a food processor, or by hand, mix oatmeal, wheat flour, salt and sugar with diced (cold) butter.
You're done when you have crumbs or pieces of dough.
Here half of the crumbs have been spread out in a buttered pie tin.
Turn on the oven, at 175 C.
Now, chop the rhubarbs and weigh them.
Chop and dice the apple.
Mix the diced apple with the rhubarbs, and the potato flour (or corn starch).
Add the chopped fruit on top of the crumbles.
Add the remaining crumbles on top of the fruits. Bake in the lower part of the oven for about 50 minutes.
Give it some color to make it crispy.
Serve with vanilla sauce or vanilla ice cream.