Trim the tough ends from the Brussels sprouts. Rinse well and pat completely dry using a clean kitchen towel or paper towels.
Using a mandoline slicer or a sharp knife, thinly slice the Brussels sprouts. Transfer them to a large mixing bowl.
In a small bowl, whisk together the olive oil, apple cider vinegar, honey, Dijon mustard, salt, and pepper until the dressing is smooth and fully emulsified.
Pour the dressing over the sliced Brussels sprouts and toss well using tongs or salad servers to evenly coat.
Add the chopped walnuts, dried cranberries, grated Parmesan cheese, chopped chives, and roasted sunflower seeds to the bowl. Toss again until everything is evenly combined.
Let the salad sit at room temperature for 15 to 30 minutes to allow the flavors to blend and the Brussels sprouts to soften slightly.