Turn the heat on under a skillet (use one with a lid) to medium–high heat. Add olive oil and butter to your pan, and let that fat get good and hot.
Chop a red onion and add it to the hot pan, then toss in the Brussels sprouts. Give them a toss, and then turn the heat down to medium-low. Add sea salt and freshly ground pepper.
Give the Brussels sprouts and diced onion another toss and then snip in your strips of leftover bacon with your kitchen shears into the pan.
Remove the pan from the heat, cover, and let it steam for about five minutes.