For the cookie layer, beat together the butter, granulated sugar, and brown sugar until creamed.
Add the egg and beat well again.
Stir in the all-purpose flour, oats, ground cinnamon, and salt (add a small splash of milk, up to 2 tablespoons, if the mixture doesn’t fully hold together).
Remove 1 cup of the mixture and set it aside.
Press the remaining dough into the lined baking pan to form a flat base.
For the fruit layer, gently toss the chopped strawberries and rhubarb with the granulated sugar, lemon juice, and cornstarch.
Layer the strawberry and rhubarb mixture over the top of the dough base.
Top with the remaining 1 cup dough, crumbling it over the fruit layer and gently pressing down.
Bake the cookie bars for 45-50 minutes, until the top is lightly golden.
Remove the strawberry rhubarb biscuit bars from the oven and allow to cool completely. Slice and serve.