In a food processor, combine the flour, brown sugar, salt, and cinnamon. Pulse to blend together.
Add the butter to the processor, and pulse until no chunks are bigger than pea-sized. Add the oatmeal and pulse to combine, but not to chop up the oats. Set aside (keep cold if you're not working very quickly here).
Toss together the rhubarb, raspberries, sugar, cornstarch, and lemon zest. Mix well, then pour into a buttered 8x8" baking dish.
Top with the oat crumble mixture, and bake for 45-50 minutes. The fruit should be bubbling, and the crumble should be turning light golden-brown. Remove and serve warm with vanilla ice cream.
Recipe by The Kitchen Paper at http://c02.ddd.mwp.accessdomain.com/raspberry-rhubarb-crisp/