Preheat the oven to 375 degrees. Grease the bottom and sides of a 9-inch pie plate or similar sized dish with 1 tablespoon softened butter.

In a large bowl, combine the rhubarb, strawberries, apricots, salt, sugar, and cornstarch. Use a rubber spatula to mix until everything is fully coated, then transfer to the pie plate.

Wipe out the bowl (theres no need to dirty a new one), then add the granulated sugar, brown sugar, cardamom, salt, cinnamon, flour and oats. Mix to combine.

Melt butter in a small pan over medium heat. Swirl it around until it foams, then keep an eye on it. It will begin to turn a few shades darker and smell nutty. Turn off the heat and as soon as it turns brown, pour it over the oat mixture, being sure to scrape out every last bit of browned milk solids. Mix until it forms a crumbly topping.

Spread the crumble over the fruit mixture to cover completely. Place the dish on a foil lined baking sheet to catch any spills, then bake for about 45 minutes, or until the top is browned and the fruit is bubbling.

Let cool for about 30 minutes, then serve with vanilla ice cream.