Preheat the oven to 375 degrees F.
Combine the orange juice and the corn starch in a small bowl and whisk to combine until the corn starch is incorporated.
Mix together the rhubarb pieces, brown sugar, corn starch slurry, orange zest, lemon zest, and cardamom in a 9x9 or 11x7-inch pan.
Combine the oats, butter, flour, pecans, pistachios, and honey in a mixing bowl. Cut with a pastry blender or forks till the mixture is combined and the butter is in pea-sized pieces.
Top the rhubarb mixture with the oat mixture and spread it out to make sure all the rhubarb is covered.
Bake for 50 minutes or until the oat crust is golden brown.
Let cool for a few minutes before serving. Serve with an optional scoop of vanilla ice cream.