Heat oil in a large skillet on medium heat. Add the onion and saute until it starts to caramelize, about 4 to 5 minutes. Add ground beef, breaking it up as it cooks for about 4-5 minutes. Add the brussels sprouts and continue cooking until the beef is cooked through.
Add the chopped apple to pan and saute for 2-3 minutes, and then stir in the lemon juice. Cook for another minute or so before seasoning to taste with salt and pepper.