Heat oven to 400 degrees.
Give the Brussels sprouts a good wash in cold water. Cut off the bottoms, then cut the Brussels sprouts in half - lengthwise. Place onto a metal sheet pan.
Chop the onion. Peel, core and chop the apple.
Add both the onion and apple to the sheet pan with the Brussels sprouts.
Cut the bacon into small pieces, around the same size as the onion.
Add to the sheet pan.
Drizzle the Brussels sprouts, onion, apple, and bacon with 2 Tbsp olive oil and sprinkle with salt and pepper. Toss well to coat everything.
Put the sheet pan full of Brussels sprouts into the hot oven.
Roast veggies for 25 mins, tossing a couple times during baking.
(You can keep cooking them 5-10 mins longer if you like your Brussels sprouts super caramelized.)
Remove veggies from oven.
Toss with apple cider vinegar and remaining 1 Tbsp olive oil.