Mix together your remaining butter, brown sugar, flour and nuts until crumbly.
Sprinkle evenly across the top.
Cover and cook on low for 4 hours or high for 2 hours.
If desired, and your crock is oven safe, place in the broiler on hi for 30-90 seconds, until lightly browned.
Let it cool for 15-20 minutes (until the topping has cooled enough to crisp up) before serving.
Do not, I repeat, do NOT walk away from the oven while your casserole is in the broiler. As little as 10 seconds could take this casserole from perfection to burnt while it is in your broiler. Trust me people, turn on your oven light and take root while you watch for that perfect golden hue to appear. Then immediately take it out to cool.