In a medium bowl, combine the sliced strawberries, chopped rhubarb, sugar, cornstarch, and lemon juice. Mix well and set aside to macerate for about 15 minutes.

Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.

In a large mixing bowl, cream together the softened butter and sugar until light and fluffy.

Add the eggs one at a time, mixing well after each addition. Stir in the vanilla extract.

In another bowl, whisk together the flour, baking powder, and salt. Gradually add to the butter mixture, alternating with the milk, until just combined.

Pour half of the cake batter into the prepared pan. Spread the fruit filling evenly over the batter, then top with the remaining batter.

Bake in the preheated oven for 40-45 minutes, or until a toothpick inserted into the center comes out clean.

Allow the cake to cool in the pan for 10 minutes before transferring to a wire rack to cool completely.