In a saucepan, combine the strawberries, rhubarb and sugar; let stand for 15 minutes. Combine cornstarch and water until smooth; stir into the fruit mixture. Bring to a boil over medium heat, stirring constantly. Cook and stir for 2 minutes. Cool to room temperature.
Combine flour, sugar, cinnamon, nutmeg, salt, baking powder and baking soda; cut in butter until crumbly. In a small bowl, beat eggs, buttermilk and extract; add to flour mixture and mix well. Spread half of the batter into a greased 9-in. square baking pan. Carefully spoon the fruit mixture on top. Spoon remaining batter over fruit mixture.
For topping, combine sugar and cinnamon; cut in butter until crumbly. Stir in walnuts. Sprinkle over batter. Bake at 350° for 40-45 minutes or until a toothpick inserted in the center comes out clean.