Preheat oven to 400 degrees F.
Clean and cut sprouts by cutting hard ends off, removing any wilted leaves, and slicing in half. Place in large bowl.
Slice red onion into 1/4 inch strips and add to bowl with sprouts.
In small bowl, whisk apple cider vinegar, honey, and whole grain mustard until well-mixed.
Pour sauce over brussels sprouts and onions and stir until well-coated.
Spray baking sheet with olive oil. Spread sprouts and onions evenly over baking sheet, turning all sprouts cut-side down.
Sprinkle with salt & pepper.
Cook in oven for 20-25 minutes, or until desired level of crispiness and char is achieved.