Spray a large saucepan with nonstick spray coating. Cook onion and carrot in the saucepan over medium-high heat until onion is tender.
In a small bowl stir together 1 cup of the skim milk, the flour, nonfat dry milk powder, and salt; stir into vegetable mixture. Stir in remaining milk and clam juice or chicken broth. Cook and stir over medium heat until thickened and bubbly.
Stir in undrained oysters, spinach, and, if desired, lemon peel. Heat through, stirring occasionally. To serve, ladle into soup bowls. Makes 4 servings.