Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish or similar size. While the oven heats, cut the rhubarb into 1/2-inch thick slices, hull and quarter the strawberries, and cut the cold butter into 1/2-inch cubes—keeping the butter cold is essential for creating a crumbly topping. Have all your ingredients measured and ready before you begin mixing.
In a large bowl, combine the prepared rhubarb and strawberries with the sugar, cornstarch, orange juice, and vanilla extract. Toss everything together gently until the fruit is evenly coated and the cornstarch is fully dissolved—this ensures the filling will thicken properly as it bakes and won't be watery. I like to let the mixture sit for just a minute or two; the fruit's natural juices start to release and blend with the sugar, creating a more cohesive filling.
Pour the fruit filling into your prepared baking dish and spread it in an even layer. In a separate bowl, combine the flour, brown sugar, oats, cinnamon, and salt. Add the cold butter cubes and use a pastry cutter, two forks, or your fingertips to work the butter into the dry mixture until it resembles coarse wet sand with pea-sized pieces of butter throughout. Sprinkle this crumble topping evenly over the fruit, then scatter the chopped pecans on top for added texture and flavor.
Bake the crumble in your preheated 350°F oven for 50-60 minutes, until the topping is golden brown and you see the fruit filling bubbling around the edges. The golden color on the oats indicates that the flavors have developed fully and the butter has created that perfect crumbly texture. I prefer to check at 50 minutes and tent with foil if the top is browning too quickly—you want it golden, not dark.
Remove the crumble from the oven and let it rest at room temperature for 1-2 hours before serving. This cooling time allows the filling to set properly so each spoonful holds together nicely, and it also ensures the dish isn't mouth-burningly hot. Serve warm or at room temperature, optionally with vanilla ice cream or whipped cream on the side.