Begin by preparing the fruit filling, which forms the heart of this cake. In a covered saucepan over low heat, cook 2 cups strawberries and 2 cups chopped rhubarb for 10 minutes.
Then add 1 1/2 tablespoons lemon juice along with a mixture of 2/3 cup granulated sugar and 1/4 cup cornstarch. Continue cooking for 8 minutes or until the mixture thickens, stirring constantly to guarantee even cooking. Set the filling aside to cool slightly while you prepare the cake batter.
For the cake base, combine 3 cups crushed Rice Chex, 1 cup granulated sugar, 2 cups all-purpose flour, 1 tablespoon baking powder, and 1 teaspoon salt in a large bowl and mix well. Cut in 1 cup butter until the mixture becomes crumbly.
Add 1 1/4 cups milk, 2 large beaten eggs, and 1/2 teaspoon almond extract, mixing until just combined.
Reserve 1 cup of this batter, then pour the remaining batter into a greased and floured 9x13 pan. Spoon the cooked strawberry-rhubarb mixture evenly over the batter, then dollop and spread the reserved batter over the top.
For the finishing touch, combine 1/2 cup all-purpose flour and 2/3 cup granulated sugar in a bowl, then cut in 1/4 cup butter until crumbly.
Sprinkle this streusel topping evenly over the cake batter. Bake at 350°F for 1 hour, until the cake is golden brown and a toothpick inserted into the cake (not the filling) comes out clean. Allow the cake to cool in the pan before serving to prevent the layers from shifting.