For the crumble topping: Combine the granulated sugar, flour and cinnamon in a bowl and mix to combine. Add the butter and cut in with a fork or pastry cutter until the mixture is crumbly in texture. Set aside.
For the strawberry-rhubarb buckle: Preheat the oven to 375 degrees F. Grease a 9-inch springform pan or round cake pan with butter.
In the bowl of a stand mixer fitted with a paddle attachment, combine the butter and granulated and brown sugars. Mix on medium speed until well creamed, 2 to 3 minutes. Add the eggs one at a time and mix until fully incorporated. Add the lemon zest and vanilla extract. Continue to mix until well blended, about 1 minute.
In a medium bowl, combine the flour, baking powder, salt and nutmeg and whisk to blend. Turn the mixer to low and slowly add the dry ingredients to the egg mixture and mix until well blended, about 1 minute. Fold the strawberries and Partially Poached Rhubarb into the batter with a rubber spatula.
Spread the batter into the prepared pan. Top with the crumble topping. Bake until the crumble is golden brown and the cake is cooked through, 40 to 50 minutes. Allow the cake to cool to room temperature.
Remove the cake from the pan and transfer to a serving plate. Slice and serve topped with whipped cream or vanilla ice cream, if desired.
Partially Poached Rhubarb:
Combine the rhubarb, granulated sugar and 1 cup water in a saucepan. Bring to a boil, stirring until the sugar dissolves. Once it reaches a boil, remove from the heat and let cool in the pan with the juices, about 45 minutes.