Cut brussels sprouts in half, then cut each half into thin pieces. Chill the onion and brussels sprouts in separate bags for 8 hours.
Melt butter in large skillet over medium heat; add pecans, and saute 5 minutes or until toasted. Remove pecans. Add onion, and cook, stirring often, 15 minutes or until caramel colored. (We even burn some of the onions.).
Add pecans and brussels sprouts, and cook about 3 minutes or until heated. Salt and pepper and serve.