In a mixing bowl, combine Dijon mustard, champagne vinegar, maple syrup, salt, black pepper, and oil. Whisk until well combined.
Add the sliced red onion to the dressing and let it sit for a few minutes.
In a large mixing bowl, add the shredded kale, red cabbage, and apple matchsticks. Transfer only the red onion to a mixing bowl, then pour approximately 3 tablespoons of the salad dressing over the vegetables and mix them thoroughly to combine.
Before serving, garnish the salad with chopped walnuts.
Pure Maple Syrup. If you're unsure about how sweet you want your slaw dressing to be, start with two teaspoons instead of 1 tablespoon (which is equivalent to 3 teaspoons), as you can always add more later.Champagne Vinegar. I strongly recommend using Napa Valley Champagne Vinegar with 7% acidity. It's the finest champagne vinegar I've ever tasted, providing a perfect blend of tanginess and sweetness.I tried a different brand of champagne vinegar, like O champagne vinegar and it resulted in a noticeably less flavorful and somewhat bitter outcome. So, if you can't find Napa Valley champagne vinegar, feel free to substitute it with apple cider vinegar or asian rice wine vinegar with less maple syrup. Please note that while substituting ingredients is a clever way to be creative and use whatever is available, it may affect the final taste of the recipe.Balance Acid and Sweetness: Keep in mind that not all ingredients are uniform. Depending on the brand or type of vinegar or maple syrup you're using, you might need to harmonize the acidity with additional vinegar or citrus, or fine-tune the sweetness by using a smaller or larger amount of maple syrup. Nutrition Facts:The nutritional value does not include walnuts, as they are an optional topping.Store Leftovers Properly: Make sure to store any leftovers in an airtight container and keep them in the refrigerator. Be sure to enjoy them within 1-2 days.