In a large bowl, toss rhubarb, strawberries, sugar, orange juice, zest, cornstarch and salt. Let stand while preparing the crumble.
Crumble Topping
In another bowl, combine flour, brown sugar, salt and cardamon or cinnamon until well mixed.
Stir in melted butter forming clumps. Stir in nuts, if desired.
Assembly
Toss fruit mixture again making sure cornstarch has dissolved in juices. Pour into a 9 to10-inch pie dish or square baking pan.
Top with crumble mixture.
Bake for 40 minutes or until fruit is bubbling and top is brown.
The mixture will thicken as it cools. Serve warm with ice cream.
Notes
1. Place baking dish on a foil lined rimmed baking sheet to prevent spills.2. Cover top with foil if browning too quickly.3. Serve with vanilla ice cream or sweetened whipped cream.