Stir brown sugar and cinnamon together in small bowl. Add butter; stir until blended then mix in walnuts. Set aside.
Lightly grease and flour 9 x 13-inch (23 x 33 cm) baking pan. Set aside.
Stir flour, baking powder and salt together in medium bowl. Set aside.
Cream butter and sugar in large bowl with electric mixer then beat in eggs and vanilla.
Add a third of flour mixture to egg mixture; beat on low until blended. Add half of the milk, beat just until blended. Continue alternating flour mixture and milk until both are incorporated into batter.
Add strawberries and rhubarb then fold the batter gently by hand just until combined. Spread batter into the prepared baking pan. Sprinkle topping evenly over batter.
Bake in preheated 350˚F (180˚C) oven until toothpick inserted near centre of cake comes out clean, 40 to 45 minutes.