Preheat oven to 350°F. Grease a 9x13 baking pan.
Mix butter, brown sugar, flour, baking powder, salt, and vanilla until crumbly. Reserve 1 1/2 cups for topping.
Press remaining dough firmly into the prepared pan. Bake 10 minutes.
Stir together strawberries, rhubarb, sugar, cornstarch, and lemon zest if desired.
Spread fruit filling evenly over the crust.
Sprinkle with reserved crumble and oats.
Bake 35–40 minutes, until golden and bubbly.
Cool completely before slicing into bars.