In a medium saucepan, melt butter over medium heat. Cook until butter turns a medium-brown color and has a nutty aroma, about 10 minutes. Remove from heat, and let cool to room temperature.
In a medium bowl, whisk together flour, oats, sugars, and salt. Drizzle with browned butter, and stir with a wooden spoon until combined. Crumble with your fingertips until desired consistency is reached.
Recipe by Bake from Scratch at https://bakefromscratch.com/apple-pie-browned-butter-oat-streusel/