Hold each brussel sprout by the stem end, and slice them until they are shredded. Give them a rinse in a colander and drain. In a large dry skillet, saute the bacon until the fat is rendered, and the bacon is crisp. With a slotted spoon, remove bacon and set aside until later. Dump most of the bacon fat into a small bowl, leaving some in the pan, and set aside. This will be your cooking oil. Do not discard.
Add your onions and garlic, and saute until translucent and fragrant. Then dump out of the pan and set aside. Add a little more bacon fat, and saute your shredded brussel sprouts in batches until they are slightly softened, but still a little crisp, and vibrant green. With each batch, add a little bit of salt & pepper, some of the onion and garlic mixture, and some of the cooked bacon. Dump into your serving bowl. Repeat this until all your brussel sprouts are cooked.
TIP: You may not end up using all of the rendered bacon fat, so that's why pouring it into a separate bowl is useful. You can add only what you need, and save some calories!