Prepare the pan(s): Heat oven to 350°F. Grease a 9″ pie plate or four 5″ tart pans with non-stick cooking spray.
Brown the butter: Place butter in a medium sauce pan over medium heat. Cook, stirring occasionally, until the solids have turned golden and the butter smells nutty, 3 to 5 minutes. Let cool slightly.
Process the dry ingredients: In the bowl of a food processor, add oats, brown sugar, cinnamon, ground cardamom and kosher salt. Pulse a few times until the crumb is medium-coarse.
Make the crust: Add the slightly cooled brown butter to the food processor and pulse the mixture until the mixture starts to clump together. Pour into the pie plate (or divide evenly between tart pans) and use a measuring cup or heavy-bottomed glass to firmly press in the crumb mixture, starting at the center and pressing down, then pressing up along the sides, using your fingers to help along. You want to use a good amount of pressure and really pack it in.
Bake: Bake for 18-20 minutes (place tart pans on a baking sheet), until the top is just starting to brown and the crust is set. If the crust puffs up after baking, gently press back into shape using a measuring cup. Cool completely before adding any fillings.