For the filling:
For the Cake:
Preheat the oven to 180° C.
Grease and flour a 9-inch springform pan.Combine the rhubarb, mashed strawberries, lemon juice in a medium saucepan.
Combine the sugar and cornstarch and add to the fruit mixture. Bring to a simmer, stirring.
Reduce heat to low and simmer, stirring, until thickened. Remove from heat and let cool.
Combine the flour, soda, baking powder, and salt in a bowl.
In a large mixing bowl, beat the butter with the granulated sugar until light and fluffy.
Beat in the eggs and sour cream then beat in the vanilla.
Slowly beat in the flour mixture until smooth and blended. Spread in the prepared pan.
Spoon cooled filling mixture over the cake batter.
Combine the flour, granulated sugar, cinnamon, nutmeg and 5 tablespoons melted butter to make the crumb.
Mix well and sprinkle evenly over the filling layer.
Bake for about 45 minutes, or until the cake is firm and topping is lightly browned.