For the dressing, combine all ingredients except the oil in a food processor. Blend until well combined and walnuts are pureed. Pour olive oil in food processor (while running) in a slow steady stream through the feed tube. Add water as needed to thin the dressing to your desired consistency. Transfer to a jar or airtight container and refrigerate.
Chop, wash, and allow kale to dry. Once dried, toss the kale with 1-2 tablespoons of the maple walnut dressing. Serve with the grapefruit, avocado, and Parmesan cheese.