Preheat oven to 400 degrees F. Place beets in a baking dish. Add 1/2 inch hot water to the bottom of the baking dish, cover with foil and roast for 1 hour. Remove the foil and let cool. Run the beets under cold water and rub off the skins. Chop into 1/2 inch pieces.Place the balsamic vinegar, water, 1/4 cup of the walnuts, raisins, mustard, garlic and thyme in a high-powered blender and process until smooth.In a large bowl, combine roasted beets, chickpeas, arugula and desired amount of dressing. Top with remaining 1/4 cup of walnuts.