Preheat your oven to 350°F (175°C). Butter or grease a 9x13-inch baking dish.
In a medium saucepan, stir together the water, cornstarch, and granulated sugar over medium heat. Add the chopped rhubarb and simmer while stirring for about 5 minutes, until the mixture thickens slightly and the rhubarb has broken down. Remove from heat and gently fold in the chopped strawberries. Set aside.
In a large mixing bowl, combine the flour, baking powder, salt, and granulated sugar.
In another bowl, whisk together the eggs, milk, melted butter, and vanilla extract until well combined.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Pour 2/3 of the batter into the prepared baking dish, spreading it out evenly.
Spoon the strawberry rhubarb filling evenly over the batter in the baking dish.
Add the remaining batter on top of the strawberry rhubarb filling, spreading it out gently.
In a small bowl, mix together the flour, brown sugar, and cinnamon for the crumb topping. Add the cold butter cubes and use a pastry cutter or your fingers to incorporate the butter into the mixture until crumbly. Sprinkle the crumb topping evenly over the strawberry rhubarb filling.
Bake in the preheated oven for about 50-65 minutes, or until a toothpick inserted into the center comes out clean and the top is golden brown.
Allow the coffee cake to cool slightly before serving.
Enjoy!