Preheat the oven to 350° F and pan spray a 9-by-13-inch rectangular pan.
For the Crust/Topping:
Mix together the flour, ground pink peppercorns, rolled oats, walnuts, brown sugar, and baking powder in a food processor. Pulse until walnuts are ground into smaller pieces - they should be granular, but don't break it down too much.Add the cold butter, pulsing until combined. Press half the oat mixture into the prepared pan, reserving the other half in a bowl for the topping. Bake for 20 minutes, or until golden brown.
For the Filling:
While the crust is baking, add the rhubarb, strawberries, lime zest, sugar, cornstarch and cardamom to a blender. Blend until smooth.
Once the crust is par-cooked, pour the filling on top of the crust and spread evenly. Crumble the remaining crust topping on top and bake for 15-20 minutes. The topping should look golden brown when done.
Let cool completely, and then cut into squares.