Partially prebake the squash to make the cutting and peeling easier. To do so, wrap the entire squash in foil and bake at 375 degrees F for about 40 minutes, or until you can pierce through the narrow end with some resistance.
Once the squash is cool enough to handle, cut in half lengthwise, and remove seeds and fibers. Cut into ¾-inch slices, peel, and then cut into dice.
Heat the oil in a soup pot. Add the onion and celery and sauté over medium heat until the onion is golden.
Add the sweet potato dice and the vegetable stock. Bring to a slow boil, then add the bay leaves, cumin, and seasoning blend. Lower the heat and simmer gently, covered, for 10 minutes.
Add the squash dice and corn. continue to cook until all the vegetables are tender, about 15 minutes longer.
With a slotted spoon, scoop out 2 cups of the solid ingredients, mash well, and stir back into the soup. Or simply insert an immersion blender into the soup pot and puree a small portion of the ingredients—enough to give the base of the soup some thickness.
Add enough rice milk to give the soup a slightly thick consistency. Season with salt and pepper, then simmer over low heat for 5 minutes longer.
If time allows, let the soup stand for an hour or so before serving, then heat through as needed.
This may also be cooked a day ahead and refrigerated, since its flavor develops nicely overnight. Stir in the parsley just before serving and garnish each portion with some thinly sliced scallion.