Pre heat oven to 350 degrees.
In a medium bowl, combine rhubarb, strawberries, sugar and lemon zest. Transfer to a buttered 8-inch baking dish.
For topping, combine flour, sugar, cinnamon and walnuts, pour in melted butter and toss to combine, tumble over fruit mixture. Bake until fruit is bubbling and topping is browned.
Serve warm with softly whipped cream or vanilla creme anglaise.