How to Make It
Preheat oven to 375°F. Grease a 2 1/2-qt. baking dish and set aside.
Coat the rhubarb in 1/4 cup of the granulated sugar, honey, orange zest, and half of the orange juice (discard the other half of the orange juice).
In a medium-sized bowl, combine remaining granulated sugar, brown sugar, flour, cinnamon, cardamom, and salt. Add butter and using a pastry cutter or your hands, blend butter into flour mixture until it resembles coarse wet sand, with a few pebble sized pieces of butter are scattered throughout. Add oats and chopped walnuts. Stir to combine.
Place rhubarb mixture in prepared baking dish. Top rhubarb evenly with oat mixture, and bake until the top becomes golden brown, about 45 minutes.