In a large saucepan combine the rhubarb, strawberries, 3/4 cup sugar, cornstarch, and lemon juice. Cook over medium heat until the fruit is soft and thickened. Remove from heat and cool completely.
Preheat the oven to 350 degrees F. Lightly grease a 9 by 13-inch pan.
In a large bowl combine 3 cups of flour, 1 cup of sugar, baking powder, baking soda, and salt. Add the butter and cut until the mixture crumbly.
In another bowl add the eggs, buttermilk, and vanilla, beat together.
Add the wet ingredients to the dry ingredients and mix until just combined.
Spread 1/2 of the batter into the prepared pan.
Spread the cooled strawberry-rhubarb sauce over top of the batter.
Spread the remaining batter over the top of the batter.
In a medium bowl combine the 3/4 cup sugar, and 3/4 cup flour, and cut in 1/4 cup butter until well combined. Sprinkle over top of the batter.
Bake in the preheated oven for 45 to 50 minutes or until a wooden pick inserted in the center comes out clean. Cool on a wire rack.