Preheat oven to 400°.
Thinly slice brussels sprouts (use a mandolin if you have one available) and toss in olive oil until all of the veggies are generously coated. Season with salt and black pepper.
Spread brussels sprouts onto baking sheet in a single layer (try not to overlap). Then sprinkle chopped onion onto the same baking sheet.
Bake for about 15 minutes, flipping halfway through. Cook time can vary depending on the thickness of your brussels sprouts. You want to cook until they are tender and just slightly browned.
In a small bowl or jar, whisk together all Honey Mustard Dressing ingredients.
Place brussels sprouts in a bowl and allow them time to cool slightly before tossing in your dressing.
Optional: Add a sprinkle of chopped walnuts.
Serve this dish warm or cold.