Preheat oven to 375 degrees F. Spray a 9" deep dish pie plate or other shallow baking dish with cooking spray.
In a large bowl toss the rhubarb and strawberries with lemon juice, zest and sugar until distributed. Spread evenly in baking dish.
In a medium bowl, whisk together brown sugar, flour and rolled oats until combined. Pour melted butter overall and mix until all dry ingredients have been well incorporated and mixture is moist and crumbly.
Cover fruit mixture completely with topping. Bake for about 35-40 minutes or until fruit is bubbly and topping is golden brown. Serve warm with ice cream.
*I used 2 Tablespoons of sugar which allowed the tartness of the rhubarb to shine through. I allowed the topping to carry the sweetness instead. If you like a sweeter filling, simply add more sugar to taste.