Preheat your oven to 350°F. While it heats, combine the strawberry rhubarb mix, sugar, and cornstarch in a large bowl. Toss gently but thoroughly until all the fruit pieces are evenly coated with the sugar-cornstarch mixture—this ensures the filling will thicken properly and cook evenly. The cornstarch will absorb the fruit's juices and create a beautifully thickened sauce.

Pour the yellow cake mix into a medium bowl. Add the melted and slightly cooled butter, then use a fork to break up the dry mix and distribute the butter evenly throughout. Work until you have a coarse, crumbly texture—it should resemble wet sand. I like to use slightly cooled butter rather than hot because it creates a crispier, more distinct crumble topping that doesn't turn muddy or compacted during baking.

Spread the fruit filling from Step 1 into a 9x13 inch baking dish, spreading it into an even layer. Scatter the cake mix crumble from Step 2 evenly over the fruit, leaving some small gaps so steam can escape and the fruit stays tender rather than mushy. Bake at 350°F for 35-45 minutes, until the topping is golden brown and the fruit filling begins to bubble slightly around the edges—this visual cue tells you the filling has thickened and the flavors have melded together.

Remove the crisp from the oven and let it rest for 5-10 minutes before serving. This allows the filling to set slightly and makes serving much cleaner. Scoop into bowls and top with a generous dollop of whipped cream or a scoop of vanilla ice cream—the cold dairy is essential here as it balances the tartness of the rhubarb and creates the perfect textural contrast with the warm, crispy topping.