Coat large saucepan with cooking spray, and heat over medium heat. Add onion, and sauté 5 minutes, or until soft. Stir in broth and 4 cups water, and bring to a boil. Add quinoa, and reduce heat to medium-low. Cover, and simmer 15 minutes.
Stir in tomatoes and spinach leaves. Simmer 5 minutes, uncovered, or until spinach and quinoa are tender. Season with salt and pepper, if desired, and serve immediately.