Brown 6 tbsp butter (cut the remaining 2 tbsp of butter into small cubes and keep in the fridge). To do this, place the butter in a small pan over medium low heat. Let this go for a few minutes, or until it reaches a deep amber hue. You’ll see it slowly start to brown throughout this process. When it's done, remove it from heat and set it aside to cool.
Preheat the oven to 375° Fahrenheit and grease a deep pie dish.
Make the filling. To do this, slice all the stone fruit. Discard the pits, and put the slices in a large bowl, along with 1/4 tsp salt, 1 tbsp lemon juice, 1/2 cup brown sugar, and 2 tsp corn starch. Toss to coat.
Now make the crumble topping. To do this, add 1 cup flour and 2 tbsp cold cubed butter to a medium sized bowl. Mix with your hands, crumbling the butter into small chunks as you go. Then add 1/2 cup rolled oats, 6 tbsp brown butter, 1/4 tsp salt, and 1/4 cup brown sugar, and continue to mix until combined.
Add the filling to your deep pie dish. Then crumble the topping over it. Bake for 40-50 minutes, or until the filling is nice and bubbly and the topping is golden brown. Let cool for about 20 minutes before serving!