Preheat your oven to 350°F (175°C). Grease a 9-inch cake pan and set aside.
In a large mixing bowl, combine flour, sugar, baking powder, and salt. Whisk together until evenly distributed.
Pour in the melted butter, eggs, vanilla extract, and milk. Mix with a spatula or whisk until the batter is smooth and slightly spoonable, with no streaks of flour remaining.
Fold in the halved strawberries and diced rhubarb gently, just until combined. The fruit should be coated but not overly mashed.
Pour the batter into the prepared cake pan, spreading it evenly with a spatula. The batter should settle with some berries visible on the surface.
In a small bowl, mix the crumble topping ingredients—flour and sugar. Add the cold butter and use your fingers or a pastry cutter to work it into coarse crumbs. Sprinkle evenly over the batter.
Bake in the preheated oven for about 45-50 minutes, or until the top is golden and a toothpick inserted into the center comes out clean. The edges may slightly pull away from the pan.
Remove the cake from the oven and allow it to cool in the pan for 15 minutes. Then, transfer to a wire rack and let it cool completely before slicing.
Slice the cake and serve it slightly warmed or at room temperature, enjoying the juicy fruit with the tender crumb and crunchy topping.