In a 4-quart Dutch oven heat oil over medium heat. Add onion; cook about 5 minutes or until tender, stirring occasionally. Add garlic and ginger; cook and stir for 30 seconds. Add the water, broth, soybeans, peas, soy sauce, and cayenne pepper. Bring to boiling; reduce heat. Simmer, covered, for 5 minutes.
Gradually stir spinach into broth mixture. Remove from heat. Using an immersion blender, blend mixture until it reaches a slightly chunky pureed consistency.
To serve, ladle soup into shallow bowls. If desired, sprinkle with black pepper and top with croutons.