Over high heat, bring chicken broth and water, mustard, and garlic to a simmer. As this is coming to a boil, de-rib and wash the spinach, then tear the leaves into 2-inch pieces. Lightly toast 4 slices of French or Italian bread. When toasted, spread with sundried tomato, pesto or green olive paste.
When the soup is at a simmer, add the chicken and simmer, uncovered for 2 minutes just to warm through. Then add the spinach leaves, and cook for a few seconds or until the leaves just wilt. Season with salt and pepper to taste.
Set 2 slices of toasted bread in the bottom of each soup bowl. Remove the soup from the heat, season to taste with salt and pepper and ladle it over the bread.