Put brussel sprouts with ends cut off and sliced in half lengthwise in a bag or bowl. Add olive oil, 1 tsp garlic salt, chili powder, squeeze 1/2 your lemon, and apple cider vinegar into your baggie or bowl. Toss with brussel sprouts.
Pour coated brussels into your air fryer basket and close lid. Cook at 390 degrees for a total of 15 minutes, stirring/flipping pieces over twice during that total time. (so every 5 minutes)
When that is cooking melt your butter and whisk in 1/4 of garlic salt, pepper, the other 1/2 of your lemon juice, and dijon mustard together.
When brussel sprouts are done pour them in a bowl and toss with this butter mixture.
Serve topped with shredded parmesan cheese if desired.