Pre heat oven to 450°F or Broiler on high.
Place the sweet potatoes in the microwave and cook for 5-8 minutes until tender.
While sweet potatoes are cooking, combine pecans, brown sugar, oats, cinnamon, nutmeg and cardamom in a small bowl.
Put the butter into a small pan and heat until butter turns brown and smells nutty.
Add to the pecan mixture to make a crumble.
Carefully remove the sweet potatoes from the microwave and place into an oven proof serving dish.
Arrange the potatoes so they are in a flat layer and season with salt and pepper
Sprinkle with the crumble and place the dish into the oven to brown the top. About 3-5 minutes.