Heat 1 tbsp avocado oil over medium heat in cast iron skillet. Add half the brussels sprouts cut side down in a single layer and cook until the cut sides turn brown, about 5 minutes. Remove from skillet and repeat with remaining oil and brussels sprouts.
Return brussels sprouts to skillet and add onion, syrup, mustard, balsamic vinegar, salt and pepper. Stir to combine. Reduce heat to low and simmer, covered, until sprouts are tender (about 10 minutes). Stir in the bacon and cook uncovered for 5 minutes. Serve warm.