Trim and discard the woody ends of the Brussels sprouts. Slice each sprout in half lengthwise. Lay each half flat-side down and thinly slice it. Gently pull apart the slices into thin strips. Set aside.
Heat a large skillet over medium heat. Add the avocado oil. Stir in the diced onions and raw diced bacon. Sauté, stirring occasionally, until the onion has softened and the bacon is fragrant and starting to render its fat, about 5–7 minutes. The bacon should be cooked but still chewy, not crispy.
Add the shaved Brussel sprouts to the skillet. Continue sautéing, stirring often, until the sprouts are tender and beginning to caramelize, about 6–8 minutes.
Pour in the sugar-free maple syrup, red wine vinegar, brown sugar substitute, water, and pinch of salt. Reduce the heat to low and let the mixture simmer, stirring occasionally, until most of the moisture has evaporated, the sauce thickens, and the sprouts are coated, about 4-6 minutes.
Remove the skillet from the heat. Taste and adjust seasoning as needed. Serve warm.