First, make the maple mustard dressing. In a small bowl or jar, whisk together the olive oil, lemon juice, apple cider vinegar, pure maple syrup, mustard, and garlic. Season with salt and pepper and set aside.
Next, shave the brussels sprouts. Use a food processor with the slicing attachment and pulse until the brussels sprouts are thinly sliced. You can also use a mandoline or sharp knife if you don't have a food processor.
Place the shredded brussels sprouts in a large bowl. Add the chopped apple, dried cranberries, sunflower seeds, and Parmesan cheese. Drizzle the salad with the dressing and toss well. Season with salt and pepper and serve.
Store the salad in an airtight container in the fridge for up to 4 days.