Heat the oil in a soup pot. Add the onions, garlic, and celery and sauté over medium-low heat until all are lightly browned.
Transfer the onion mixture to a food processor or blender with 1 cup water and process until smoothly pureed, then transfer to the soup pot. Or, simply add 1 cup water to the pot and process with an immersion blender until pureed.
Add 3 cups water and bring to a rapid simmer. Whisk in the nut butter, about ⅓ cup at a time.
Stir in the curry powder, cumin, nutmeg, lemon juice, and orange juice. Bring to a rapid simmer, then lower the heat. Cover and simmer gently for 15 minutes.
Stir in the steamed vegetables. If the soup is too thick, add a bit more water. The soup should have a slightly thick consistency.
Season with salt and pepper, then serve hot or allow to cool to room temperature. Garnish each serving with a sprinkling of scallion and, if desired, chopped nuts