Preheat the oven to 375°F. Line a 9x9-inch pan with foil hanging over the sides. Spray with cooking oil.
Combine strawberries, rhubarb, white sugar, cornstarch, and lemon juice in a bowl. Toss and set aside.
Combine the rest of the ingredients except for walnuts in another bowl. If mixture is too wet add a little more flour. Press half of the mixture into bottom of pan.
Spread fruit mixture on top.
Add walnuts to remaining crumb mixture. Crumble on top of fruit mixture evenly.
Bake for 35-40 minutes, until fruit is bubbly and top is golden.