For the cake:
For the fruit filling:
For the crumb topping:
Preheat oven to 325 degrees F. Butter a 9-inch springform pan, line the bottom with parchment paper, butter the parchment paper, and dust the entire pan with flour.
To make the crumb topping, mix all ingredients with a fork in a small bowl until combined. Set aside.
To make the cake batter, whisk together the sour cream, egg, egg yolk, and vanilla extract in a small bowl. In the bowl of a stand mixer fitted with a paddle attachment, add the flour, sugar, baking soda, baking powder, salt, and butter. Mix on medium high speed until combined and mixture has come together into a crumbly dough. Add the sour cream mixture in three additions, mixing after each until just combined.
Spread the cake batter into the bottom of the prepared springform pan. In a medium bowl, combine the sliced strawberries and rhubarb, sugar, cornstarch and ginger. Toss to coat and scatter on top of the cake batter. Use a fork to break the crumb mixture into large chunks and sprinkle over the top of the fruit mixture.
Bake for about 1 hour, or until a toothpick inserted into the center comes out with a few crumbs and the crumb topping is golden brown. Let cool in the pan for 10 minutes, the remove the sides and cool completely on a wire rack.
The coffee cake can be stored covered in the fridge for several days.