In a 5-quart pan or Dutch oven, heat oil over medium heat. Add onions, garlic, carrots and celery. Cook about 10 minutes or until vegetables start to soften. Stir in pepper and salt along with enough vegetable broth to completely cover the vegetables, about 4½ to 5 cups. Bring to a boil.
Stir in frozen peas along with remaining vegetable broth and return to a boil. Reduce heat to low and simmer 10 to 12 minutes, or until peas are tender but still bright, vivid green.
Stir in mint, lemon juice and lemon zest. Remove pan from heat.
Using a regular or immersion blender, puree the soup in batches.
Ladle into bowls and garnish with a dollop of light sour cream or plain yogurt, lemon zest and a mint leaf, if desired. Refrigerate leftovers in a tightly covered container.
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