Preheat the oven to 400 degrees.

Using a wide-bottomed pan over medium heat; add butter, sugar, and sherry and mix. Once the butter begins to melt, add the rhubarb to the skillet. Stirring occasionally allows the rhubarb to soften and cook down slightly. The sugar mixture will give a really nice flavor to the rhubarb.
After 5 to 7 minutes when the rhubarb has softened, turn off the heat and gently fold in the strawberries.

Meanwhile, using your hands combine the flour, oats, brown sugar, walnuts, salt, cinnamon, and cardamom in a bowl. Once everything is combined add the cold butter and using your fingertips gently break up the butter and rub it together with the flour mixture to create the crumble topping. Place in the fridge to keep the butter nice and cold until you are ready to bake the crumble.

When you are ready to bake the crumble, place the rhubarb and strawberry mixture into an oven-safe dish. Spoon the crumble topping directly onto the top of the fruit mixture. Place the dish in the oven for about 45 minutes. I like to place the crumble on top of a baking sheet with parchment paper to catch the bubbling fruit that drips over the pan. Once the crumble topping looks golden and the fruit is bubbling out from the side, remove from the oven and serve with your favorite ice cream.