Preheat oven to 400°F.
Combine the diced squash, onions, vegetable oil, brown sugar, sage, fennel seeds, cinnamon, red chili flakes and black pepper. Toss until the squash is well coated with oil and spices. Transfer to a parchment-lined sheet pan, scraping out all of the oil and spices onto the vegetables. Use 1 sheet pan for 32 servings, and 2 sheet pans for 64 servings.
Place in the preheated 400°F oven and bake for about 40-60 minutes, or until the squash and onion are caramelized and tender. Rotate the pan half way through cooking time. Remove from oven and set aside.
Transfer the caramelized squash to a 1 or 2 gallon container. Add the water, soymilk, Kikkoman Ponzu Citrus Seasoned Dressing & Sauce, salt and ground nutmeg. Use an immersion blender to blend until the sauce is very smooth. Pour sauce into a shallow pan and chill until ready to use.