Add the onions, Brussels sprouts, and garlic, and sauté for about 10 minutes, stirring occasionally, until the sprouts are lightly browned and very tender.
Add the vinegar, salt, and pepper, and continue cooking for about 5 more minutes, stirring occasionally, and reducing the heat if the sprouts are getting too deeply browned.
Slow Cooker Directions
Add Brussels sprouts and onions to the slow cooker, toss with oil, garlic, vinegar, salt, and pepper, and stir to combine. Cook on low for 5 - 6 hours or on high for 2 ½ - 3 hours. (Slow cooker cooking times may vary—Get to know your slow cooker and, if necessary, adjust cooking times accordingly.)