Melt butter in a medium saucepan over medium-low heat. Cook onion about 10 minutes or until lightly browned and soft, stirring frequently.
Blend onion, peas, spinach, lemon juice, thyme and 2 cups chicken broth in a blender or food processor until very smooth. Return to same saucepan. Stir in remaining broth. Gently heat about 5 minutes, stirring occasionally.
Stir in cream, if desired. Heat over low heat about 1 minute. Do not boil. Garnish with garlic croutons, if desired.
Variations: Prepare recipe as directed, except: For Minted Sweet Pea Soup, use 1/2 cup fresh mint leaves instead of thyme in Step 2. For Tuscan Pea Soup, add 1/2 cup fresh basil instead of thyme in Step 2. Garnish with Parmesan cheese with garlic croutons in Step 3.